In my experience with Xflows, it is much more likely to be a plug lead, no.1 culprit, followed by a dodgy plug, followed by the cap being spark eroded by a loose plug lead onto it (central lead from the coil). Over nearly 8 years of ownership, I have had 4 errant missfires, each more peculiar than the last, each finally down to one of the above! So, I change the leads every 2 years now.
If you can get the car to me in Newbury, I'm more than happy to have a look at it with you, perhaps even give you some hints/advise on fettling these great engines!