Hotpoint WD64 Prog C problem?

I have a Hotpoint WD64. It appeaes to work on all other progs except C. When selecting C it does the usual Tumble of clothes for a couple of mins then advances the timer knob to midway between C and D, All ok so far. Then it waits a couple of secs and then advances to D. WHY???? When using any other prog it does the usual tumble of clothes and then advances halfway to the next prog and then proceeds to do that wash cycle correctly. Only on C will it not stop and do the wash cycle correctly.
Anyone any ideas?????

Thanks