A large-ish stick got wedged into my rear mech, locked up the wheel and bent the cage that holds the jockey wheels away from the main mechanism. I've managed to bend it back to a certain extent and re-adjusted the indexing but it ain't perfect anymore. Is it knackered?

Probably, they don't last forever, especially when that happens!!

I share your pain, last week similar happened to me. 1x destroyed X0 mech, 8 spokes and gear hanger. Couldn't afford to buy another X0 so got an X7 for £30 from Merlin. Fitted it all last night and then realised that the cassette (three rides old PG990) is also bent! Going to stick with SS for a bit!