Well, yesterday and Saturday, actually.

Cam belt, cam cover gaskets, water pump, thermostat, etc. were changed 2K miles ago and last weekend it started leaking coolant.

Immediately assumed it would be the thermostat as I used one of the housings with the thinner type flange.
Stripped it all down and pressurised the system to 1 bar with an airline. Water dribbling out slowly around the crank pulley area but also towards the nearside side of the sump. Hmm.
Thermostat, then? Area around the thermostat appeared bone dry, though. Poking around with a bit of rag on the end of a wire under the joint with the transfer pipe revealed absolutely no moisture also.
Eventually spotted what looked like a little leak around the water pump housing while poking around with a mirror but still wasn't convinced that it wasn't running down behind the backplate from further up.
Decided to pull the water pump off and when I did the sealing O ring looked like it had been on there for 10 years.

Completely flattened and a texture like plastic. Now, this was a pattern pump. Can't remember where it came from as it had been kept as a spare for a couple of years and it was only changed because the coolant was being changed anyway, and the original had 120k on it.

I reckon the O ring was a bit sub-standard.
Back together with a new genuine seal, refilled with coolant and on pressurising the system the leak appears to have been fixed.

Drove to work today and no sign of any leakage.
So much for changing the water pump with the cam belt to save time if it fails later.
