Reading from various sources, it seems that what happened is that the Longannet power station (about 2400MW capacity) up in Fife, Scotland, had an unexpected shutdown, and this was followed within 2 minutes by another unexpected shutdown of Sizewell B nuclear in Suffolk (about 1200MW).
The resulting loss of capacity brought in automatic load shedding to stabilize the network. Somebody on the IEE forum noted from UPS logs and the National Grid's realtime network page that the grid frequency had dropped right down to 48Hz.
