NO HEATING AND NO HOT WATER?

I HAVE A POTTERTON PUMA 100E.
MY PROBLEM IS I HAVE NO HEATING OR HOT WATER
I HAVE FOLLOWED THE HEATER DIAGRAM AND IT LED ME TO THE HOT WATER FLOW SWITCH AND THE WATER PRESSURE SWITCH SO I CHANGED BOTH BUT STILL THE BOILER WON'T IGNITE AND WHEN I SWITCH ON THE HOT WATER TAP A RED LIGHT GOES ON, ON THE MODULE BOARD, I CAN HEAR ALL THE SWITCHES CLICK ON WHEN I TURN THE BOILER ON AND OFF, THE TAPS ON AND THE CENTRAL HEATING ON. I CAN HEAR THE FAN WORKING AND THE MOTOR WORKING WHEN I CALL FOR HOT WATER. THERE IS NO HOT WATER PRESURRE WHEN THE TAPS ARE ON.
IS THERE ANYBODY WHO CAN HELP PLEASE?