Final cleaning of the cylinder heads was undertaken with carburettor cleaner and a tooth brush; all of the nooks and crannies are now free of carbon deposits.
A final scrape around the thread of the notch nut to remove some 'paint' residue exposed this little problem - the reaction between the steel nut and aluminium tube had dissolved the casting for roughly two thirds of the diameter. The casting was too weak for any attempt to undo the nut; a replacement pipe will be needed. A temporary repair was effected by fabricating an aluminium sheet collar and bedding it in ProWeld resin inside the casting. The repair is invisible from both the outside and inside of the pipe.
Here is an internal view of the corrosion. 'A' highlights the inside of the steel notch nut - the thread has completely rusted away.
It would be advisable for every owner of a PIII to check their outlet pipes as those illustrated could have failed at any time.