Salopian:
You said:

"If the chamber had been lengthened here in the UK it does not have to be reproofed by law."

Is this really true? I have always assumed from what I heard on this BB that any lengthening of a gun's chambers AUTOMATICALLY required it to be reproofed before it could be sold legally.

I would probably be tough to determine where the gunsmith did the work.

To all:
I would be using low-pressure loads in any case.

The only constant in life is change.