So finally got round to fitting the factory hose from water pump to thermostat on Thursday. Significantly tighter on both thermostat and pump connection stubs than the one from the (allegedly) good quality, but not factory, set I fitted shortly after getting the car. The cable operated spring clip compressor thing worked fine when fitting the new pipe to the thermostat end but couldn't do the deed at the water pump when working from underneath. Probably be OK if working from the top. So the pump end got a Mikalor stainless steel band and bolt type clip. Needed a bit of careful alignment to keep the bolt head clear but went on well in the end and stayed in place whilst ratcheting up the bolt head. Jubilee worm drive types don't stay put and are the very devil to do from underneath!
Test run and top up, no leaks despite encountering the airlock issue for the first time ever. Sixth time unlucky I guess. Still dry all through standing round Friday. Last check at about 9 pm.
Yay! Fixed. I thought. Celebrated too soon.
Wandered out on the paper collection walk this morning and ... territorial marks under the car. Perhaps 1/4 or 1/2 a teaspoons worth. Quick look confirms the joint at the (new) thermostat is slightly wet.
So it seems that it does it overnight. Presumably due to temperature changes but how that would generate enough pressure to produce a leak is beyond me. In retrospect its always had odd days and periods without marking its territory overnight and, so far as I recall, its always been overnight. Never when parked up during the day.
Getting to the point where I'm about ready to abandon my stance against bodges and chuck a tin of sealer jollop in. But which one! I'm sure it had jollop in the cooling system when I bought it but I assumed that was to seal up the sadly bodged water pump gasket. Must have been good stuff as its the only time I've ever been able to see where it settled when sealing gasket. That one was so bad it probably leaked under simple pressure head when the coolant was poured in.