Found since getting Helga back on the road that she was misfiring under boost. At some point I’d taken the distributer off and lost the timing. Simple to sort normally, but what made things more confusing is that the timing marks have been machined off due to the flywheel being lightened. There were three lines of paint but no indication of which one was TDC. Eventually sussed it with the old screwdriver down the #1 plug hole trick. It was the yellow line!
She’s running great now, though one potential issue I spotted is that pressurising the vac capsule to around 15psi only retards the timing three degrees, whereas I thought stock it should be more like six, so from 16 to 10 degrees. Currently it only goes back to 13, which I think might be causing the boost to be slightly erratic in how far it will go. This is the opposite of what I was expecting because I was told when I bought the car that the distributer had been modified to retard further on boost, so the off-boost timing could be advanced more than stock.
In the middle of all this the bonnet cable snapped at the end that attaches to the pull, so for now that’s been replaced with some mole grips.
While I was diagnosing the misfire I did a compression check, the results seem good enough so the head gasket swap can wait. I’ll end up doing it eventually because I’d like to replace the timing chain and tensioners, plus I’ve got an uncracked exhaust manifold ready to fit. Of course that’s not necessarily a head off job but I’ll bet at least one of the studs will shear.