In recent months, sterling has tended to follow the fortunes of the euro, falling against the dollar as the single currency fell. Up until a few months ago, the pound was much more likely to track the dollar, as many investors believed the British economy had more in common with the US than with Europe. Intervention is relatively rare and tends to be used only as a last resort. Central banks have a pool of cash set aside for the purposes of supporting the national currency - in the case of the European Central Bank, that amounts to around 50bn euros. In this case, central banks in the eurozone, the US, Japan and Britain got together and each bought euros on the foreign exchange market, propping up its value.
Intervention is often unsuccessful because of the sheer size of the currency markets.
It will only work if investors are looking for an excuse to buy the currency anyway. In this case, the euro had already strengthened slightly earlier in the day and so intervention was going with the market flow.