[Less]

I think I've worked out what happened to cause the bang.
As I said I've used this method before and all turned out well. I think what caused this was that the eggs I was using were very large so when I removed the saucer to check and found them only half cooked back went the saucer which due to the heat of the water would have condensation on it making a seal around the mug turning the whole thing into a pressure cooker with no vent.

Now if I can just turn my mind to cleaning up I can try again and prove the theory.