Hi Colin...welcome to the Forum
It seems like a lot of guesswork has been taking place so far....the only way forward is by using proper diagnostic tests (does it fuel, air going in/out the engine, fault codes, wrong fuel, etc)
I would definitely entrust a VAG specialist in your area to look at it logically. Fault codes would be a good place to start. Looking at the Fuel Filter housing / supply to see what fuel it is drawing up (if any) is another good place to start with.
Let us know how you get on with it
Cheers Trevor