Cheers for the quick responses guys, they're much appreciated. I'll have a look at the cap and neck before I go to work in the morning. With regards to the engine cover moving, would I still need a new cap? Or is it down to something else that needs to be fixed/replaced?
Check the engine cover mounts and rubber bungs... time allowing, take the engine cover off and make sure all the mounts are screwed in tight, placing a small amount of grease on the bungs for easier removal next time.
If you haven't removed the cover before, make sure NOT to put pulling pressure on the bottom as it could snap. Like most things, once you've done it a couple of times, it's easy.
Remove the induction pipe from the Engine Cover / Air box (bottom right), then unclip the engine cover from near the MAF housing (top left), there's a top and bottom of the clip, pull the outlet of the MAF housing left, away from the engine cover, then pull the top right on the engine cover upwards and left, essentially prising the cover from the mounts... this I've found is the best way.
Since I made sure all the mounts were on and tightened my oil cap top hasn't seperated, worth checking mate.