It would seem that my guesses were more or less correct – the largest earthquake in the UK for 25 years. Apparently most of the country felt it, so I think everyone knows how unnerving it feels! The BBC has an initial story on it here. The BGS site, which is responding a little better than it was at 1 am, also has a technical report on the quake. The BGS also has more general information on UK seismic activity.