From UK Border Agency’s website:

Notice for holders of expired visas whose travel plans have been disrupted by the volcanic ash

17 April 2010
The UK Border Agency would like to reassure travellers who have been unable to leave the UK and whose visas have now expired. We appreciate that this is due to exceptional circumstances beyond your control.

Please ensure that you retain proof of planned travel that would have enabled you to leave the UK before your visa expired. Please only contact the agency for further advice in exceptional circumstances. We will update this advice as the situation develops.