It became clear what had happened when I dissembled the angle grinder. The four machine screws that secure the spindle assembly to the gearcase had worked loose. This had allowed the spindle to partly disengage from the pinion stripping the pinion teeth in the process.
It wasn't a particularly cheap angle grinder so I reckoned it was worth saving if at all possible. And after a quick internet search, and £3.50 later (the cost of a replacement pinion), it's working again as of this morning!
I'm wondering if the vibration you often get when working with cup brushes fitted to angle grinders is what caused the screws to loosen? So that I don't get held up again when an angle grinder goes down I've bought a second, cheap (£17) angle grinder as backup. What I'll maybe do is use the cup brushes with the cheap angle grinder and keep the better angle grinder for working with flap discs, grinding or cutting.
Anyway - work resumes on the Saloon as of this afternoon!