Following the second 'blowing' at the exhaust/manifold join I decided it must be a design fault where there is no support for the exhaust pipe except right at the back. That means that all the vibrations and movement of the engine are transmitted to that joint.
Yesterday I visited Richard Stewart (the designer of the car) to pick up some spare parts, and we talked about the problem. He was very surprised that the rear of the exhaust was bolted directly to the chassis and told me it should be rubber mounted to avoid that exact problem. I then realised that in the box of spare parts (aka 'bits left over') that the previous owner had given me there was a rubber mounting that should fit the bill. (He has subsequently admitted he maybe should have fitted it).
So the solid mount was changed for a rubber mount.
Took her for a run and was very surprised to hear a dramatic reduction in the noise level. I wasn't expecting it but I guess it is logical, as the rubber mount prevents the noise from being amplified by the body of the car.
It remains to be seen if my next attempt to fix the exhaust leak lasts longer then the previous one.