There was a third, match-based, option too!

I was really thinking it would have to be that £324 replacement but someone in the club put me onto Priory Magnetos whose owner let me into the secret about how apparently stuck ATDs will move if they are turned by the engine. I don't recall any issues with them on my two BSA A50s which are the only other ATD units I've had experience of - usually I have manual advance/retard levers. Mine was genuinely stuck but of course after rinsing it out with WD40, oiling it all up so that it worked in my hand and discovering the lateral movement, when I put it back on the bike and it appeared stuck I assumed it was just too knackered to work - but it isn't!

Once again, the value of an owners club is obvious. On that note the end of the BMRiders forum last year was such a huge loss to owners of BMW bikes of all older varieties. Very sad.