Popping in exhaust

Thanks for the responses everyone! I have checked the wires and they're gravy, looks like the next step will be to pull the plugs on that side and make sure they are not fouled and then run a compression test (which I am afraid to see the results of!). I'll take a look at the headers to be sure they are not leaking, but wouldn't that be rather noisy if there was a leak?

I'm thinking it's probably dirty valves, but what makes you think it's #7, Crossram? Is that cylinder prone to deposits?