After the trip out this morning, the first long distance since fixing the coolant pipes and thermostat, you can imagine my feelings watching the temperatue needle slowly climb and fall around the 2/3 mark. And, having arrived home, the top cap went, and the whole thing overheated, big time.
Refilling it, and starting the engine up resulted in more steam and smoke than I've ever seen. The little puffs of blue smoke I've noticed recently have gone. Now it's clouds and clouds of steam and smoke. An inspection of the oil filler concluded the diagnosis. The only other place with that much mayo is a jar of Hellmans 
So, the head gasket, it appears, has gone. Ouch
