Right, an update, such as it is... didn't manage to do anything to it at all at the weekend, unfortunately. Had a rare Friday-night 'session' of an order of magnitude not seen or equalled since the days I was actually riding this bike 20 years ago... So the last thing I wanted to do on Saturday (and even Sunday) was have my sore head inches from an angle grinder for an hour. Will have a look at it this weekend instead...