Misfire diagnosis

[Colin Wilks]

Have you done a compression test to exclude sticky valves or bad rings? If the engine has sat doing nothing for some time (like my recently acquired one) I have found things tend to get gummed up.
Edit to add apologies, your post says running ok in December, but maybe worth testing anyway - one person's "running OK" being another's "dog rough" (that's English for c**p).