OK, Camcoat did their wonderful thing and left me with a manifold coated inside with a ceramic insulator, and outside with a dissipator. The effect is to maintain energy in the gasflow and help control under-bonnet temperatures. And the pipes cool down much faster than otherwise too.
The car then went on the back of my friend Norman's transporter to visit Alastair at Stainless Creations in Falkirk. Despite his slightly dodgy website and its distinct impression that everything is about 4.5" pipes on Scoobies, the bulk of his work is the sensible (ie affordable) repair of exotic German exhausts.
We talked, he listened, and I left it to him, The result was a single big-bore pipe to a simple box that has left me at about 98db at sound checks. I have already recommended him to others.
Now I'm exhausted, and need to sit down....