I was using a 3T low profile trolley jack (Clarke, I think).
Having thought about it, I think what might have happened is the wheels of the jack got stuck, so as the car went up, the jack head pulled forward until 'bam!' it came off the jackpoint on the car.
I'm definitely more tempted to get some ramps, but a bit worried about getting the car up on to them without going over the top!
Update from today: car recovered to indy. Mechanic has had it running and reversed off the lorry. No warning lights showing. He thinks the fluid might have just been rainwater collected on the top of the tray, as coolant level is fine (he checked). So, hopefully the only thing needed replacement might just be the tray, but should know more tomorrow when he gets it up on the lift.