just got back from my old tin top i have a problem with it miss firing and sputtering quite badly.
so tonight ive taken all the top off to get to the plugs all of them have a tiny bit of oil down them apart from one which seemed to have quite alot on it and the tips were really black am i right in thinking this i due to the rocker cover gaskets gone the car is a vectra gsi 2.5 v6

On the V6 the rocker cover gaskets are prone to leaking. The oil in the plug holes makes the ht leads break down and causes the misfires. You'll need to sort the oil leaks first as well or you're gonna have a vicious circle/groundhog day.
Cavweb forums has a lot of info and how to's on it on this very subject.
Good luck.

As above --
Fix the oil leak first

Usually replacing the plugs or leads alone isn't enough -- need to be changed at the same time.
On a lot of engines this sort of problem can even take out the coil packs.

The cam cover gaskets aren't too bad a job but changing the HT leads is a right pain