Why isn't my boiler igniting when it has been programmed to?

[bryanb]

My Potterton boiler is set to come on at 6.30am and has been alright up until the last few days.

Recently I wake up to find that the thermostat is set okay, the programmer in the kitchen is on and set okay but the central heating has not come on & the rads are cold!! I look in the Garage and the boiler isn't running, so I press the ignition switch in and it lights straight away but why isn't igniting it it when it is programmed to?

Also a rough idea of estimate of cost for this query would be appreciated.

 

[Min]

If it is a combi boiler the water pressure may have fallen too low.