I was talking to my late brother on the phone one day in the early 80s - he in Camborne, me near Helston, when my brother said "did you just hear that bang"? A few seconds later I did hear a big bang. Concorde was still entertaining us with sonic booms most evenings in those days, but this was the wrong time of day, and in any case this sounded different.
We later heard that there had been a minor earthquake in the area (less than 4.0 on the Richter scale) but I discovered a large crack in my concrete water tank which had developed a leak.
Perhaps there was an earthquake in the vicinity - have there been any reports?