There are two causes:
A timer button has been pressed and held for more than 15 seconds, or a timer button has become stuck.
Check buttons for freedom of movement (if necessary remove any rubber covering by gently prising out). If all seems well, try turning off at mains for 30 mins. then turn on and set timer if possible.
If not possible to set timer because C03 still showing, then press a timer button (often there's just one with the left/right arrows marked on or above) and hold it for more than 180 seconds (yes, really!)
If this does not work then the timer has developed a fault that cannot be corrected and should be replaced. Personally, I would try the above a few more times before you pay £ 95.00 + fitting costs, though....just a thought :)
Kitman, March 2010