My Central Heating not working but I am getting hot water?

The Potterton Puma 80, fires up when the hot water demanded however just recently the central heating does not turn on. You can hear something inside making noise as if the pump is working but the gas does not flame up with the boiler is set to constant setting. There is some noise coming from boiler which is unusual not heard during normal operation.

Please can you help?

Alpen
May 2008