Done now. (just got to refill oil and test).

Used Loctite Blue Maxx sealant (dunno if it's available in UK), supposed to be about the best available.

Put a thin sliver above and below gasket and a reasonable bit on the end of the half moons. The old gasket hadn't been fitted properly and had a gap at one end, so if nothing else it shouldn't piss oil everywhere any more.

iirc, oil pressure dropped to about 20psi at idle when hot (or maybe just under).

Can't remember what it was when revved but I have checked it before and was ok.