I'm now a bit worried the mating face of the crank-case is the problem.
I had previously tried new gaskets but it still blew fuel/air out from behind the reed-block (blue arrow below) during cranking.
In the absence of some Engineer's Blue and a proper surface table I rubbed the inlet manifold on some 1200 grit wet-and-dry atop a sheet of 6mm safety glass on a 'flat' surface; it seemed flat with no high-spots or dips apparent.
I couldn't do the same with the reed block because of the rubber inserts for the reeds, but using the edge of a steel rule didn't show up any problems (which would have to be significant to show up using that test!).
The bolt nearest that area had a "stripped thread" (wouldn't tighten fully and slipped) but (thanks to your pdf) it now has a bolt of the correct length and the hole has been re-tapped (no room for a helicoil). It now tightens to the specified torque (as do all the others).
A bit of Hylomar on either side of the gasket is probably last chance saloon for what is a twenty yr old engine for which supplies of spares are dwindling - or completely dwindled.
On the plus side, I can now disassemble the inlet stuff blindfold, single-handed and in under three minutes!