Hey Owain- how'd you get on with this...?
By chance mine did what sounds like same thing, was running sweet until one day a couple of months ago, the guys I was working with asked how it's going so I told 'em "sweet"....on the way home, it started idling unusual, so fueled up, (but didn't need it)....but was still fine except for occasional miss at idle....until it died completely a couple of weeks ago
Long story short, the maggie seemed to check ok, and still occasionally start up, but not for long, so I took the maggie out to the guy who rebuilt it a year or so ago,for a check over....
To me it kinda showed symptoms of dodgy slip-ring spike, but he just called me and said the new slip-ring he used has a hole blown in it, but not at the spike, so he replaced it and I'll collect it tomorrow and see what happens...I'll keep you posted