Had a look at lunchtime. Bit rushed but won't have another chance until the weekend. Not certain, but room for guarded optimism.
Drained the coolant and then refilled it, because I wanted to get an idea how much was left. Howling gale blew up while I was pouring it back (What are the chances, eh? Well about 100% IME) so I lost a bit, but still had some left over. This suggests:
(a) Coolant leakage/loss, if any, must have been fairly limited.
(b) This engine suffers from air pockets when refilled (with hose "burping"). Not good, but it might go some way to explain the "now you see it, now you don't" coolant, assuming I wasn't just hallucinating.
Took the plugs out and turned the engine over. Plugs weren't wet and nothing spat out of the plugholes.
Wedged a funnel in the radiator filler and started the engine. It did blow bubbles, and hadn't entirely stopped when the coolant started to steam/I ran out of time, but it wasn't violent and I already knew for a fact there was air in there, so it may not be evidence of a head gasket problem.
Also encouraging was the fact that the exhaust appeared to be blowing from where it joins the exhaust manifold. Too inaccessible when hot to be sure but IIRC one of the bolts was a bit unconvincing due to thread corrosion, and might have let go, causing failure of my home made gasket. If that's the case it SHOULD be a fairly easy and cheap fix.
So maybe I'm the duck that cried wolf. We'll see.