Drove to Stromlo, got the bike out of the car, faffed around a bit then went for a warm up. Bike felt weird and it turned out the wheel was rubbing on the chainstay. I checked out the thru-axle and it was right, but realised my tyre was losing air. Took the wheel out, put a tube in, kept rolling around but it still felt weird. Rolled a few tracks to get down to the carpark again, checked the spokes - loose. Tightened the spokes, rode it around the carpark, it was a bit better. Tightened them some more, felt better still but still wasn't right. A mate came over, I explained the problem, he pushed on the bottom bracket and there was heeeeeeeaps of flex. Then he said "oh, you're missing a bolt".
Turns out the bolt holding the chainstay on at the back end had fallen out. Cue panic, as I had an advanced skills course on in 10 minutes and tear-drop shaped nuts aren't in my spare kit bag. Fortunately, it had dropped out just behind the car - grabbed the bits, stuck them back in (tight) and now am going to buy some loctite.