I believe that the fact that it will tow start rules out the immobiliser, but take a look at the EWS. This is what the owner and I found on a local car:
https://www.the75andztclub.co.uk/for...d.php?t=312290 post 4.
One pin is missing.
Hope you've checked the spade terminal on the starter solenoid.
Regards
PS I've no idea what side O/S is, left & right much easier.