My brother knew a bloke who was a welder at Haverton Hill at the time. There was considerable industrial unrest at the time and he told my brother that, unless they were closely supervised, they welded by stuffing the area to be joined with welding rods, then welding a skin over the top so that it looked like a sound weld. As they were paid piece work, in effect, it meant that they got more work 'done' in a given time. Obviously welds were inspected, but, how realistic is it to expect that every inch of a weld is tested.
It doesn't argue against the cause of the sinking, but it does speak volumes for the quality of the build.