Update!
I now have a working French Horn again!
Didn't get a chance to do any more investigating so took it to the CT for a minor fail to get another couple of legal months to sort it.
Kept pressing the button all the way to the test centre just in case, but no joy - nothing at all, right up until parked outside.
Heard a faint beep from somewhere deep inside the test hall but just assumed it was another car. Tester returns 30 minutes later, 'All OK, nice car' and prints out the report...
I didn't dare say anything just in case he'd forgotten to test the horn, but once safely out of earshot the horn is definately working - and has done all the way home!
Thanks everyone for their input, at least I've watched so many videos about the clockspring and the MFU that I could probably do it in my sleep now if I have to
I know that MGs have their idiosyncrasies but this is ridiculous!
What's that black canvas thing behind the seats for?