The bitch bolt gave in without the slightest struggle. Wheel off, extension bars out to an axle stand and a breaker bar augmented by six feet of t 6061-6 alloy tube which used to be a strut on an aeroplane. I pulled firmly on it and the bolt came undone. The exhaust manifold studs look like they will give up so tomorrow I would very much like to have the head off for checking. Fingers crossed.