Well, it worked pretty well. The's a bit of a leak at the manifold end but liveable with, and the pipe seems secure. The header tank end is definitely a bit wobbly, but it's the adhesion to the new copper pipe that's not good (that should have been the easy bit!) so I'll probably have another go at that.
However, it is serviceable and good enough for a 'get out of trouble' fix. And I can run the engine here on the mooring so the normal routines can carry on.