Sorry, i was off line for a while there.
Edward, no you were right the first time - it is the long filler pipe that fits into the aluminium rear engine casing that has been damaged by some previous owner/dealer, and because of that the filler cap doesn't fit peoperly. This means that the filler cap isn't sealing around it's o-rings, and therefore I'm guessing will have an impact on the vaccum performance?
So, I'm trying to figure out how to get the whole filler pipe out to either repair or replace it. Looking at some of the earlier responses, it sounds like it is just a tight fit in the aluminium rear engine case and that I need to twist and pull it.....and then it should come out....
Cheers lads and sorry for any confusion!