Swapped motors and the patient is now at home standing in the corner, I've told it to have a jolly good think about its behaviour and deliberately stood the old faithful 40+ next to it so hopefully it will get jealous as it sees me keep going out with that! ( I do think psychology important in cases like this )
Haven't had time to strip it yet but have noticed something concerning. The fuel cap has a very small leak, more a weeping, but with the motor tilted I think the fuel has slowly been leaking and dripping onto and presumably into the ignition.

Obviously I will soon find out, but can anyone think of anything that fuel will have / could have dissolved or otherwise ruined in that area?