NO HOT WATER BUT CENTRAL HEATING ON MY SUPREMA 60L MODEL?

I CANNOT GET THE BOILER TO FIRE WHEN CALLING FOR HOT WATER,IT WORKS FINE FOR CENTRAL HEATING WHEN THE ROOM STAT IS CALLING,I HAVE BY-PASSED FAULT BY TURNING ALL RADIATORS OFF AND SETTING THE ROOM STAT TO HIGH AND SETTING THE TIMERS ACCORDINGLY,BUT WOULD LIKE TO RESOLVE PROBLEM CORRECTLY,CAN ANYONE HELP?