Some people really ram plugs into sockets, if the plugs are not proper BS 1363 as many imports are not, this could easily crack the front which is very hard and brittle.

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

Is it only a particular brand/model, or have you tried other brands/models and found a similar result?

Sp33dy said:

Some thoughts are this is due to over tightened screws

Sometimes, particularly if flush-fitted, tightening down on an uneven surface, or if a metal backbox has become somehow distorted.

Newfutile said:

if the plugs are not proper BS 1363 as many imports are not, this could easily crack the front which is very hard and brittle

If a socket-outlet has undergone type tests to the correct standard, BS 1363-2 does include some very rough treatment tests, and cover 15000 insertions and 15000 withdrawals, by machine, at a rate of 6 insertions and 6 withdrawals per minute, as well as ball pressure test for abnormal heat conditions.

If a product has not undergone those tests, then it should not be marked in accordance with the standard ... and of course no guarantees as to the performance.

Let me add a little more context for this one, this has occurred on sockets where a pluggable wi-fi extender device is used. The wi-fi extender device is also under investigation.

I've only looked at two sockets so far, the one pictured and a really old (est 30+ year old) one, all pictures seen from reports show a similar crack. thus far there's been about 20 or so reports of this.

Sp33dy said:

Let me add a little more context for this one, this has occurred on sockets where a pluggable wi-fi extender device is used. The wi-fi extender device is also under investigation.

I would be inclined to check the dimensions of the plug-in product against the dimensions of BS 1363-1. For a quick check, Electrical Safety First had a 'plug checker', and perhaps consider its weight etc. against tests in BS 1363-1.

indeed that is next on the list of things to test, I've a BS1363-1 plug / pin gauge in the lab.

Maybe there is some movement or expansion in the pins on the Wi-Fi extender due to heat.

if you suspect it is getting hot then temperature stickers that have a 'high tide' mark can be used to support  or disprove this. Its the sort of thing we use a lot of inside kit to show if users have been keeping vents clear and so on, as it can affect  who pays for repair.

Mike
PS

Various temperature spans are available - but the 'A series' 40C to 70c one is most useful, and maybe the B series 77C to 115C  anything much lower goes black in the shirt pocket prior to installation, and much over 115C everything electronics is toast anyway.

good point Mike thankyou, will look to get some of these inbound.

I had a similar issue recently.  See attached picture

The culprit was a Hot Tub.  Which claims it uses 2050W at 20°C

thus

2050w / 230v = 8.91

Now factor in the usage for 8 hours and the plug and socket arrangement starts to warm up.