Hi Billy,
I got the spark plugs from k-tec think they were quite expensive maybe around 50 - 60 quid odds for 4, just changed the leads cos i had a spare bit of cash and theyre relatively cheap anyways, thought it couldnt do it any harm.
Im sure your local renault garage has the o-ring. Im sure theres only one! but really you should be okay taking it off - when you put it back get some silikone grease on it to stick it in place and youll be fine.
Cant remember what size of socket it was - it was one of them special ones with the rubber inside them for removing spark plugs. Would go and check, but im working offshore near Saudi at the moment so a bit difficult......
cheers!