06 plate citroen berlingo 1.9 dielsel milage fault?

My van has covered 22000 miles since purchase from new in aug 2006, i noticed my trip meter was showing really good milage for the ammount of fuel i was putting in.
I put £40 worth of diesel in yesterday and set my trip to zero, started the van this morning and my trip was now showing 842 miles, I selected from trip to odometer which now shows 23,842 miles. Over night the milage on my van has shot up 842 miles without moving, any common faults or does it need a new instrument cluster. Thanks for any help