Just to make sure the plug and the plug lead weren't the problem I swapped them over from cylinder 2 and 3. And in fact the problem also moved from cylinder 3 to 2. In the end it turned out the spark plug was knackered.

The only thing i'm worried about is whether the unburned fuel from running on 3 cylinders has contaminated the oil. It has been running on 3 cylinders for a few minutes I guess, mostly idling.