What is the matter with my Potterton Puma 80 Combi Boiler system?

I've Recently moved into my new flat and have been unable to get the boiler system to function properly. I am finding that no hot water is getting into either the radiators or coming out out of the hot water taps.

The thermostat in the hall seems to light up green when the boiler switch is set to "timed" but not when swithched to "constant" (in fact when switched to constant it appears to be off altogether). I can hear a click from the thermostat, but when this happens the boiler seems not to fire up or infact do anything at all bar have a little green and orange light on (no motor noise, or burner ignition flame).

Is this a common/simple problem and should I get someone out to fix the system, or will I be able to resolve this myself? (bearing in mind that i have limited, if no, experience of these sort of things!!).

Regards from a freezing new home owner in edinburgh

Mike.