Broke down a couple of days ago, coil pack went. The AA guy came (and luckily had a compatible replacement with him), and while the engine plastic cover was off, he pointed out that there's a lot of oil around, and seems to be coming from the filler.

My filler cap doesn't seem to make a good seal, so there must be some vapor escaping.

My question is, should there be a washer of some kinds on it?

I looked at a listing on ebay for one, and in the exploded parts picture, it shows a washer type thing. There isn't one on mine...
Should there be? And if I buy a new cap, will it come with the washer/ring?