Ok solved my own problem, easy when you suss it, using this as source
http://www.webupd8.org/2009/11/get-list-of-packages-and-dependencies.html

On the offline machine in terminal

sudo apt-get --print-uris --yes install python3-pyqt5 | grep ^\' | cut -d\' -f2 >mydownload.txt

this creates a list of package urls required for python3-pyqt5 into a "mydownload.txt" file

copy mydownload.txt file to a folder on an online linux machine, open folder in terminal and use wget to download packages

wget --input-file mydownload.txt

now copy this folder with the downloaded packages (6) back to the offline machine and open folder in terminal and run

sudo dpkg -i *.deb

bingo, all packages required by electrum 3.0.3 installed without having to put the machine online

Thanks for the replies 
Short update

Just tried a new install of Manjaro on offline machine, same error as with Mint, guessing your machine must have done an apt-get at some time!

Off to try the other suggestions, just wish 3.0.3 would simply work like 2.9.3 did!

I have an air gapped machine running Mint 18.1, Electrum 2.9.3 runs fine.

No 3.0.* version of Electrum will run (./electrum or python3 electrum), all throw up this: Error: Could not import PyQt5 on Linux systems, you may try 'sudo apt-get install python3-pyqt5'

I obviously do NOT want to connect, the Electrum readme says:

If you downloaded the official package (tar.gz), you can run
Electrum from its root directory, without installing it on your
system; all the python dependencies are included in the 'packages'
directory. To run Electrum from its root directory, just do::  ./electrum

Has anyone managed to do a standalone non connected install, if so, how?

Cheers