Steve,
Legally you are entitled to charge the full amount !
( Discharge of contract - Specific performance)
However it is probably better, to include in your terms & conditions a cancellation fee if they fail to give, say, 48 hours notice.
Better still, take the longer view. by all means make them aware that it has an effect on your livelyhood so that they feel some guilt but be gratious about it - be the nice man that they let down but will be the one they call next time require a carpet cleaned.
John.