I know the type with the filler neck are better, and bolt straight on, but also cost a fortune. I was quoted 65 - 75 pounds at Detling. I have a filler neck. What is the story with using a bike fuel cap and a separate neck?

I have an extra fuel filler neck.

I don't see why not as long as it fits, doesn't leak and locks

ideally if your going to track or race the car its best not to have a filler cap on the bodywork due to the fact it might get broken off but if your just going to use on the road it will be ok as long as it seals properly and has a tether to stop you losing the cap