Hmm, I'm not sure it's this either. As I say - the plugs were changed on Saturday - we didn't notice that they were soaked in fuel or anything when we took them out.
I had just driven the car there though and it hadn't sat around long, though was probably sat for an hour or so, which is long enough by the sounds of it. Hmm.
I'm wondering if the coil pack was damaged or something when the plugs were changed, but I have no idea how to go about checking it.