I think i've reached the end of the road with this search. My main 12 word phrase does not seem  to work in either mycelium, electrum, bitaddress.org. Not tried Multibit; but I'm sure it won't work there either. I'm gutted; and also annoyed because I know it should work.

The thing I found strange was that my other 12 word phrase that worked for Electrum and revealed historical transactions also worked in  Mycelium.
 
I'm going to park this problem for a while till i get fresh ideas; but thank you to everyone that helped esp. nc50lc and ETFbitcoin.
Thank you.

Thanks again nc50lc. I raised an issue https://github.com/spesmilo/electrum/issues/7893. The responses have been very interesting although somewhat disheartening. They're saying that prior to version 2.0 Electrum used a fixed word list; and if my word list is not in that list it could not have been generated by Electrum; so basically I must have used a different wallet to Electrum.

Well, I know the date of purchase 100% during 2011 or before 2011.
I know 100% I used a wallet that generated the 12 word seed which I wrote down and found in my note books.
I know 100% when I plug those seeds into Electrum they're accepted by Electrum without issue; but wallets display zero balance.
I know 100% I successfully transferred coins between the wallets over a very short period of a week or 2 max and then never touched the wallet or Bitcoin again until 2016 or so. By then my PC were long broken and got rid off.

I'm now trying to understand whether my 12 word seed was generated by a post 2.0 version of Electrum or some other wallet type.
Does anyone have an idea what wallet type that had the same look and feel as Electrum existed in or before 2011?
There has to be an answer to this mystery. Let's solve it.

Thanks for your suggestion nc50lc. I tried your suggestion but electrum crashes whether restoring wallet or creating new wallet the following error:
 python electrum.py create
Password (hit return if you do not wish to encrypt your wallet):
in order to use wallet encryption, please install pycrypto  (sudo easy_install pycrypto)
server (default:ecdsa.org):
port (default:50000):
fee (default 0.005):
if you are restoring an existing wallet, enter the seed. otherwise just press enter:
Your seed is 12f2458bab0372ecc4d2097f0df39167
Please store it safely
Traceback (most recent call last):
  File "electrum.py", line 715, in <module>
    wallet.create_new_address(False, None)
  File "electrum.py", line 295, in create_new_address
    secexp = ecdsa.util.randrange_from_seed__trytryagain( seed, order )
  File "/usr/local/lib/python2.7/dist-packages/ecdsa/util.py", line 193, in randrange_from_seed__trytryagain
    extrabyte = int2byte(ord(generate(1)) & lsb_of_ones(extrabits))
  File "/usr/local/lib/python2.7/dist-packages/ecdsa/util.py", line 98, in __call__
    a = [next(self.generator) for i in range(numbytes)]
  File "/usr/local/lib/python2.7/dist-packages/ecdsa/util.py", line 109, in block_generator
    ("prng-%d-%s" % (counter, seed)).encode()
UnicodeDecodeError: 'ascii' codec can't decode byte 0xb6 in position 7: ordinal not in range(128)

I tried to reach out to thomasv; however it said he does not accept messages from newbies. So I'm wondering if one of you guys can point him out to this post.
I feel like we're getting close to solving this mystery.

Great comments from ETFbitcoin, and yes it's a line from Kubla Khan, and as I said; I was using it for practice purposes only; sending and receiving small amounts of bitcoin. I don't remember the exact steps for purchasing the coins; but roughly, back then; when you purchased the coins from bitcoin.org I think, you then had to transfer them from your purchase account to a wallet. I'm 99% sure that the wallet was electrum. When setting up the wallet it generated the seed automatically; but it offered you the option to enter your own 12 word seed and that I remember very clearly.
I used my favourite poem at the time and words from a favourite song (not shared that yet). For my main account I used default seed generated by Electrum.
It also said that you must never write the words anywhere in case someone discovered them and used them to still your coins. I wrote them down anyway and forgot about them for years until my interest was aroused by the sudden increase of Bitcoin value; then began the long search for the seed. I turned the house and garage over and under looking for them and eventually after  years  of searching I spotted them, scribbled so nonchalantly it's a wonder I ever found them. The excitement was electric. I could barely breathe when I typed them into electrum; but my heart sank as the BTC balance came up as zero.
Then began the search for proof of purchase. You see; if I could get the date of purchase and amount I paid I could at the very list go to blockchain explorer and locate my address and my coins because they would still be on the blockchain; with very few transactions clustered around the date of purchase. Needless to say, I was not able to find any proof; banks only keep records for 6 years so they could not help either. So I was back to square one. The only thing I have is a bunch of stupid meaningless words; but potentially worth a fortune. So I'm not ready to give up yet.
The latest breakthrough was finding those 12 words with the 10.7mBTC. That proved to me that the seed works under the correct circumstances.

1. Yes it's possible I downloaded a dodgy Electrum copy and someone stole all my coins. Then let's find the trail on the blockchain. I don't think this happened because nobody cared about Bitcoin to want to steal it back then.

2. Electrum wasn't available before 2011, nobody but  spesmilo knows that for sure; his first commit on github is bumping up the version number.
from distutils.core import setup
version = "0.29"
version = "0.30"
So question, might be when did github to allow developers to push their code first come out. He might have been using SVN before that.
3. If not electrum then what other wallets were there, in shall we say 2011. Just bitcoin core and electrum right, I don't think there was anything else. I suspect I might have used a beta version; but honestly I don't really remember.
I know 100% that I bought the coins before or at the latest 2011; because I was working for this one company and I left that company in 2011 and I had already bought the coins by then.

I have now managed to check all my seeds against version 0.3 and most of the words are rejected. So  now back to square one.
Maybe it was a different version of Electrum, maybe it was some other random wallet type whose name I can't remember. What a nightmare.

Thank you all once again. I liked nc50lc's response which explains what I suspected was happening with my original recovery seed phrase; returning zero BTC. Just to clarify a bit more. I have at least four or 5 very old electrum wallets pre-2011; all of which are displaying zero BTC on recovery and now thanks to one of you I know why. One of those wallets is the one with the 10.7mBTC as I've been corrected to say and for which I shared one of the transactions and one of addresses in my last post.
Am I 100% sure that the wallet belongs to me? Yes definitely; who else would it belong to unless, it's the first private key collision on the BTC blockchain ever. I read somewhere that it would take brute force until the end of the Universe to come up with the same seed or to  hack the private key from the public key. If I managed to do that; then I would be hailed a genius or best hacker on the planet. I'm neither; I can assure you that.
Now that's settled; I will try and follow the suggestions in the link posted by nc50lc (thank you) over the next few weeks.

Just to prove my point, if one of you has the time and inclination you could try and recover the coins in this old electrum wallet given by this old seed. (weave circle round thrice close your eyes holy dread honey hath jan) I reckon it could have anything between 8-12 BTC or have been recently recovered and emptied as I shared it already with a recovery agent in the states. I used the addresses to practice sending and receiving BTC to myself just to understand how the process worked. Remember I'm talking about something that was very novel and didn't make much sense back in 2009,2010 or even 2011. Do I have a tonne of BTC out there that I can't get hold off, because I was too negligent and stupid. Damn right I do.
I don't actually mind that the whole world will now know these particular seed phrases; as I want proof that they once held my BTC and that they must trace back to the original address with at least 800-1500 BTC. Does that make sense?

Wow, I'm impressed by all the responses and kindness of everyone that responded. Thank you all so much, you are amazing people.
To address the questions raised. I bought anything between 500 and 1500 BTC back in 2009 or 2010, can't remember  the amount exactly. I downloaded electrum on my PC, and spent a couple of days teaching myself to transfer between wallets. I wrote down the seed words for each wallet in a note book and once I was successful in making a few transfers I totally forgot about Bitcoin; until 2016 when the price of Bitcoin started rallying like crazy.

Then I spent 5 years searching for the notebook and eventually found it. I restored the wallet but it was empty and showed no transactions ever taking place in the history. That is still the case with that wallet. Then yesterday I was paging through a notebook, for a novel I wrote (Two River Farm and the Cry of the Jackal if anyone is interested); and here I found another set of 12 words. I promptly plugged them into electrum and was pleasantly surprised  to see historical transactions on it; but depressed to see the balance as zero BTC.

Since the chances of someone else having the same seed as me as virtually zero, I can only imagine a scammer; somehow got hold of my seed, quite possible as I was using a windows XP laptop at the time and withdrew all my Bitcoin.
I think I'm in way over my head here; despite more than five years of searching.

Here is one of the transactions, see what you make of it and let me know.49b17aaeb9308d2d314438baf6321033af09f62c0e9914e5b01ac4d7ba481a4d  bc1qmuuc762cw02gg9j7fmqrw0wcmr0ffm5xkxjnln          10.78134

Either my money has been stolen or we've found the first ever key collision in the history of Bitcoin and that's worth the Bitcoin Foundation paying off my kids' student loans and some.

Any comments are fully appreciated.

I found my lost seed and recovered my electrum wallet; only to find that wallet had been emptied.     10.78134 BTC gone, now 0. Can anyone explain to me how that is possible. How did someone successfully hack my account.