A colleague at work lent me his endoscope, a little camera used to look through the spark holes into the cylinder. It was a cheap thing from eBay so the video feed was unclear but I did see that some sort of liquid was sitting in the cylinder closest to rear of the car. Not a lot, but still enough to give me confidence that my piston rings are ok.
I then bought a compression tester and eureka! Only one cylinder has compression.
The cylinder closest to the gearbox reads 7.5bar (it's supposed to be 8.6!)
The cylinder closest to the rear (which had the liquid) gets absolutely no compression, none at all.
So I'm hoping it's just a blown gasket or bad valve timing.
At first I was a little surprised that some liquid was still in the cylinder, but it makes sense... it's not been burned up by any ignition and there's not enough of it to be simply pushed out.
In any case, it's time to take it all apart again