Thanks David, that gave me the confidence to be a little more persuasive and I got them out, just waiting for the new ones and I'll see if that helps the issue at all.
I also read that faulty or worn crankcase ventilation hoses can cause rough idling / misfiring issues - does that seem likely? They seem cheap enough to give it a go, but looking on rimmerbros I have to buy ones depending on the engine number. I know where the engine number should be but it seems impossible to actually see it without taking things apart to get to it?