The message from Mr X contains these words:
Oooh, yes. I remember that one. When I was about 15 a friend and I came by about ten gallons of petrol that had had a bit of diesel mixed in it. So we waited for a very still evening and sploshed it all round a test-digging where the M26 now runs near Otford. Set it off remotely after about five minutes for it to evaporate 'cos we'd heard about fuel-air explosives and wanted to try one for ourselves. Damned lucky we were nearly 100 yards away. Some of the saplings it snapped off must have been nearly 2" thick.