Ha! Thanks. Basically, during the ice age most of northern Britain was covered in ice (reaching around the Thames Valley area). The melting of this has has caused the ground to raise slightly over thousands of years due to the release of the weight of the ice. Meanwhile the ground in the south is lowering slightly as the ground in the north rises. Occasionally you'll get earthquakes due to this movement, which is why the UK gets earthquakes despite the nearest plate margin being in the Mediterranean.
How can you be sure that you weren't the only one who could feel the "slight rumbling"' and that the whole thing wasn't due to your gas-tronomy?!!!
