Overheating is often accompanied by yellowing, or indeed blackening of the plastics - this cold crack  effect I have seen where long (vacuum cleaner I think) cables were in use and then they were being pulled tight or tripped over - could it be that ?

Unless the box behind is over full (too shallow, whatever) and the socket is riding the cables, or the wall is very lumpy, I'd struggle to see how that effect would occur with normal tightening.
I suppose you could try and  find out how the sockets are used or if there is some killer appliance in use. - funny things can happen sometimes. Overheating in 13A plugs usually follows the line of the fuse.
As an extreme example this mains supplied food serving cabinet was 'in spec' at 2400watts but the  (still reading low resistance)13A fuse in the plug clearly got quite hot and bothered.  ('it's stopped working' does not capture the full horror, does it?) I suspect the fusholder clips were not in good contact.

 
Mike

Overheating is often accompanied by yellowing, or indeed blackening of the plastics - this cold crack  effect I have seen where long (vacuum cleaner I think) cables were in use and then they were being pulled tight or tripped over - could it be that ?

Unless the box behind is over full (too shallow, whatever) and the socket is riding the cables, or the wall is very lumpy, I'd struggle to see how that effect would occur with normal tightening.
I suppose you could try and  find out how the sockets are used or if there is some killer appliance in use. - funny things can happen sometimes. Overheating in 13A plugs usually follows the line of the fuse.
As an extreme example this mains supplied food serving cabinet was 'in spec' at 2400watts but the  (still reading low resistance)13A fuse in the plug clearly got quite hot and bothered.  ('it's stopped working' does not capture the full horror, does it?) I suspect the fusholder clips were not in good contact.

 
Mike