Thanks for the further information you four - much appreciated.
You're confirming that the frame really is not worth the trouble - I've taken off all the components to keep as spares.
DannoXYZ - I think you have a definite point - that bit of the frame seems too sparse, cut into and machined not to be steel, and half-thickness to accomadate the derailleur too(!) ; I reckon that's very plausible it's a weakness.
The bike's 6 years old, done good service for daily commutes on London's roads, which are like ploughed fields here and there - I'm going to get a new one.
Thanks all!