It looks like the line terminal was not making good contact, as the terminal itself got hotter than the fuse to which it connects. It could have been as simple as a loose screw, but was probably aggravated by the heat conducted from the fuse contacts.
Runaway thermal damage is a known problem with less-than-pefect BS1363 accessories subjected to prolonged low overload. The fuse will do a very good job protecting against an overload of 30A by blowing quickly. Less so with a load of say 18A, which will cause prolonged heating. A good quality accessory installed properly will probably last the course but if there is any weakness about the fuse contacts, or the termination is not 100%, it sets off the progressive increase in resistance that leads to burnout.
Avoidance of prolonged low overload has always been a good design goal as it is hard to protect against without massively over-engineering things.