Re my last post.
I have ordered a new set of 4 coil packs, but in the meanwhile I bought one locally to replace what I believe to be the main culprit.
Now I know why it is playing up, it can't be replaced as the torx screw holding it in is completely stripped, no doubt it needed replacement some time ago; but the old owner stripped the thread and never changed it as the screw can't be removed.
The torx head is female and the centre is completely rounded, no room for a dremel to create a slot, so how do I get the bugger out? Looks like a garage job and money I can ill afford.