And the answer. Firstly, thanks again to everyone who suggested what was going on. Here’s what the problem was, and how we fixed it. And I say "we" because I wasn't dumb enough to turn down Andrew's extraordinarily generous offer. And had the added pleasure of his company for a day. And now he knows his way around plastic inlet manifolds... ;-) The inlet manifold gasket was shot. The metal face wasn't eroded. The new gasket from Rimmer Bros worked fine. No additional sealant was used. Two diversions along the way. I’d previously been misled by where the drip occurred, see above. And after the new gasket was fitted… it continued! But that was due to a surprising amount of coolant sitting in the curved rib at the lower edge of the sump (corrected, see below). And a bit of hunting when it was all reassembled, which was cured by excising the damaged end of a vacuum hose. Could I have done that myself once the fault was known? I’d have needed a couple more tools and an awful lot of remote handholding. Jonathan PS: The test drive was fun. Took a load of stuff to the recycling centre. Temperature fine. No drips. And then on the way home the temperature gauge dropped off the low end of the scale… loose connection on the sender! :-)