Ha, only gone and broke it haven't I ?
The output knob was sticking a long way out of the fascia, so I gave it a gentle push (yes, a gentle one) and the control shaft crunched straight through the back of the pot. I suspect it's had a knock at some point - there wasn't a great deal of resistance to it being turned before.
Oh well, another job to do when I re-cap it
Best it happens now, at home, rather than later, and it's not like I don't enjoy a bit of tinkering