Right, we're on track to sorting it! I originally couldn't see any signs of a leak. Not sure why, as when I topped it up, a very obvious drip started beneath the car. Basically, one of the pipes (annoying, one in a difficult to get to location) has obviously sprung a leak. I'll need to get it jacked up tomorrow, find the leak, patch it up and then take it for a flush. Cheers to DavidP for the advice, too!
Once I'd put in some cheapy Halfords stuff (just to move the car, to hell with driving it hard for now) and let it run for a minute it seemed to move fine. It felt like the gearbox was slipping a little, but nothing like what it was doing yesterday.
Unsure, wasn't me that changed it last.