Plug Electrical fire - what to do ?

Wire come lose on the rear of the socket? It's cracked anyway so swap it for a new one

Looks like the fuse didn't blow cleanly. I'd replace the plug and socket and make sure you have the right rating of fuse for the appliance. If something had really drawn too much current the MCB should have tripped, you do have a modern RCD protected consumer unit? Assuming you do I'd check the MCBs are B rated, they are normal for domestic installations and trip more quickly when detecting an overload.

Looks rusty in there so maybe condensation allowing current to track between pins??

Fuse hadn’t blown as was still switching on which is very odd . It does look corroded or heat damaged

Replaced the socket and the plug. All wires were tight and no sign of damage at the rear of the socket.

Could have running a tumble dryer and washing machine on the same socket at the same time be the issue?

Loose connections can do this, it maybe too late to tell but is the fuse loose in its carriers?

Could the fuse have rusted from being damp then the increased resistance of the rust caused the heat issue?

is the fuse a real one and not a fake?

Could the fuse have rusted from being damp then the increased resistance of the rust caused the heat issue?

Most likely this.
Corrosion causes the resistance to increase which eventually leads to very noticeable heating.

Very much looks like the fuse carrier was loose and poor contact. Once contacts overheat it only gets worse.

I'd be sending the plug and photos to the product safety manager at the appliance manufacturer, assuming you likely never had any reason to change the fuse or even pop open the carrier. Probably also copy trading standards, not in expectation they have the resource to follow up, but at least it's on record.
There's too much badly made crap these days in the name of lowest possible cost.

assuming you likely never had any reason to change the fuse or even pop open the carrier

I do remember having changed the fuse at some point as the original 13amp fuse had blown. The 13amp fuse I replaced it with was out of a packet I’ve had kicking around for some years.

I can’t believe that doesn’t trip the consumer unit though

Looks like there is a similar albeit much less significant scorching on the other socket too…

A slow over current won't always trip fuses or rcd's. They're designed around sudden increases in power (eg a full on short).

I’ve seen this numerous times in my work life.
It is a poor quality plug that was the intial cause, either the pins were slightly too small, or, from the picture, the fuse holder is rubbish.
There was no reason for the circuit breaker, or fuse to trip/break, as there would never have been more than 10 or so amps going through it. The poor connection has caused local heating around where it was loose, and thus the heat damage visible.
Cut the plug off, buy a good quality one (not 50pence from a corner shop), change the socket outlet, and all will be good.
The Manufacturers of the appliance will probably not be interested at all. It was a faulty appliance that caused the Grenfell fire, yet the cladding people are getting the blame as their cladding was not fire resistant. I think they should be aiming more toward the appliance makers, as their stuff can be rubbish, as witnessed above.
In the electrical world we are now being driven to fit Arc Fault Detection Devices, currently it is only in high rises and HMO’s, but in a couple of years it’ll be in every house (they are £100+ each). This is not to protect against faults in the internal wiring, it is to protect against appliance faults, again, to me, not sorting out the cause, but letting the Manufacturers of appliances off, and allowing them to sell their goods that only just meet any safety standards,and probably do fail it (1), but we have so little inspection of these things, no one bothers.

(1) 10 years or so ago there were a number of consumer unit fires across the Country. At the time, consumer units for domestic premises were plastic, and should have been fire retardent, or, self extinguishing. 7 of 10 tested by the London Fire Brigade (iirc) failed the test, and just kept on burning. Yet all of the consumer units had the relevant CE/BS marks on them showing they were to that standard. There have been no prosecutions of the Makers of these deficient parts, even though they knew they were selling plastic that wasnt suitable. Now, domestic consumer units are metal, so they cannot burn.And of course, the Makers charge more for them, and clearly make more money due to their initial failings at making things to a British Standard.

Presumably that tumble dryer plug has been previously used in the less scorched socket at some point?

Has condensation been running down the wall and getting the plug/socket wet?

Did you take the plastic fuse cover off or was it already missing?

Could the fuse have rusted from being damp then the increased resistance of the rust caused the heat issue?

This...

It never draw more than 13A (or not much more), so the fuse didn't blow, nor the MCB.

However, the resistance of the connection in the plug increased enough to start getting very hot, which caused the damage.

Could have running a tumble dryer and washing machine on the same socket at the same time be the issue?

Yes - a lot of (cheaper) double sockets are not rated for 2x13 Amps. They assume some level of diversity, one of the plugs used for something low power. Some of the better makes (MK I think) are rated for 26 Amps total

Yes – a lot of (cheaper) double sockets are not rated for 2×13 Amps.

I don't believe that at all, they wouldn't be allowed to be CE marked if that were the case, nor sold in the UK.

Yes – a lot of (cheaper) double sockets are not rated for 2×13 Amps….
I don’t believe that at all, they wouldn’t be allowed to be CE marked if that were the case, nor sold in the UK.

It’s true. A double socket outlet is BS rated for 13 amps total, not 13 amps each side. Some makes, as said above, have a higher rating. The British Standard tests them at up to 20 amps, but rates them as 13A total. (not sure if that is 10 each side, or 13 and 7amps for the test)

14 amps + 6 amps

From BS 1363-2 : 2016

Test current (A), test voltage (V): 14A+6A, 250V~ for 2 gang

Seems barking mad not to rate them to 26A, given that's the max load they can be expected to sustain.

Question,

Is the scorching on the other socket because you've had the tumble dryer in both sockets at different times, or is the washing machine plug looking similar?

Is it that the fuse hasn’t blown when it should have?

Fuses blow due to excess current, not excess heat.

Fuses blow due to excess current, not excess heat.

Well they actually blow because of excess heat as that is the method by which they work....

Juts it'll be several hundred degrees for a fuse to blow...

Pedant. 😁

@alanl thanks for the info on Arc Fault Detection Devices, if I get any electrical work done I'll get one fitted.

Weirdly, just had (today) exactly this on the dishwasher.

5 year old Bosch dishwasher in an MK socket (new 6 years ago when we did the kitchen). Other half of the socket (fridge freezer) is perfect as is the plug. The bosch plug is actually slightly melted. the area is

New plug or new dishwasher?

Was the fuse actually open to touch or was there a cover you’re removed in those pics?

I can’t believe that doesn’t trip the consumer unit though

The CU will only trip if the current is huge (on a ring main >30A) or if there is a difference between the current flowing in the live and neutral (ie. it appears to be "leaking to earth").  What you have here is a current flowing - but <13A or the fuse would have blown, through a point of high resistance generating lots of heat.  I wonder if a smoke detector in the utility room would have detected it?  Perhaps not enough - but if it had got worse it would.  Given washing machines and tumble driers are relatively common causes of fires it would make sense to have an actual smoke detector there.  I suspect most people don't - I will probably do that now.

I wonder if a smoke detector in the utility room would have detected it?

Probably not if the OP didn't smell it himself.

This has reminded me to check the plug on my Bosch dishwasher, though.