I have a BCH balance, but can't retrieve it

Hello there,
To try and give as much info as possible - here is the background to where I'm at now....

First bought bitcoin in early 2014, have been using blockchain.info as my wallet (with 2FA & all the extra security)
Back then, HD wallets weren't around. 90% of my bitcoin was held in a single address.
Years later, I log into BC.info and get told that HD wallets are here & would I like to upgrade to one - and transfer any old addresses into this HD wallet. Yes, I do that.
In hindsight, I now realise BC.info imported this old address - but I don't think it fully 'transferred' the funds into the HD wallet - it merely became a custodian for the address, if that makes sense.
The recovery phrase for this old address, was more of a hash for the password to it (eg way more words than are used in a modern, 12-word wallet seed)

So before I start anything with trying to claim my Bitcoin Cash (BCH) - I decide it would be prudent to create a new wallet & move all my coins there. That way if any website for BCH is dodgy, I will only be exposing my wallet seed for a now empty BTC wallet. (but I'm figuring the transaction history will still be there to prove my ownership of BCH)
When I move my funds, I notice I have to do 2x separate transactions - one for the 10% of my coins held in the new HD wallet - but almost as a separate 'balance' I have to do another transaction for the 90% of my coins that were from the old (non-HD) address. I couldn't do it in one go. Anyway, that's done now. Coins safe.

I have used BCH block explorers (like Blockchair) and they confirm that I hold a positive BCH balance from that old BTC address (that had 90% of my coins in)
I first use the BTC.com tool, that was mentioned in a recent blog post. This worked fairly successfully, in that the 10% of my BTC was credited into BCH (which was newer, and from the HD wallet format) - but it didn't credit the other coins in that old address.
So then I try downloading the Electron Cash wallet - and using the private key (that I can get from BC.info) I use the tool for Wallet/Private Keys/Sweep - and I get a message back saying there are no inputs.
I also try using the Android app Coinomi - and much the same thing happens, I use the private key and I get told Zero balance

So how can I have a positive BCH balance on block explorers (which confirm I had a funded wallet pre-fork) - and I have the private key for this address.... Yet I still can't get access to my BCH

If anyone can give me some pointers, I would very much appreciate it!
Thanks for reading

In fact - first correct answer to this, that results in me getting access to my BCH (and I will probably have to check back tomorrow, now) - I promise that if I can successfully get my BCH - I will send that person $50 worth of BCH (at whatever the exchange rate is when it's solved).