Went down into the kitchen and assailed by hot electric smell. While seeking the source of the smell, there was a very loud bang and the kitchen ring main breaker tripped. I could see a very black socket and this plug.
Yes, that is daylight all the way through.
I'm guessing that the fuseholder got slack and caused a high resistance and due to the load (tumble dryer) gradually heated up the plug and the fuse blew. Although quite why the fuse blew I have no idea. But with enough vigour to blast out both sides of the plug.
The socket is being replaced as well.