Hello Huw,
Where on earth do a I start? Good question since the simple circuits of years ago have been supplemented by control units which control things we would ever have thought of. By what you say, it could be reasonable to suspect the indicator stalk, but as Trevor says any of the associated contacts - including the earth of 'what on earth!' may be at fault. Not being one to give up on DIY cures, but access to curcuit information, metering equipment and experience of such issues on certain models are really essential requirements. Let's hope someone might come up with a solution, but if not, sometimes a hour of an auto electrician's time might be a wirthwhile consideration.
Not sure what part of S. Wales you are from, but there is a good one man band auto electrician in Bridgend - of course there will be others.
Hoping you get some more advise on how to cure it.
Kind regards,
Gareth.