All stripped and now I really am puzzled! It took me ages to lift the barrels as they were absolutely solidly stuck down to most of the gasket. The yammie bond on the bottom had bonded to the cases in most a, but the front is completely blown out, and the bottom LH side was on the way out. I've attached pics and you can see what I mean. Oddly, the LH side bit that was partly out was stuck down solid to the cases and I ripped a bit trying to lift it off as you can see in the pic.
Cases are a matched pair, surface was dressed and made flat before fitting new studs, studs were correct length, casings hadn't lifted around the studs (I'd slightly countersunk them before fitting), gasket glue had bonded, most of gasket absolutely solid, base of barrels seem flat...... ???
all I can do is rebuild I suppose :-)