My boiler stops working in cold weather?

I have an Ideal Classic NF60 Gas fired boiler, the boiler is reliable all year until we have cold weather snap and then it stops working. The room stat, timer and pump all function and the light within the boiler control box is alight to indicate the system is "calling" for the boiler to ignite - but it doesn't. The boiler will randomly ignite and if you flick the on off switch on the control box a few times it will sometimes ignite. As soon as warmer weather, even mild winter weather returns, the boiler starts to function properly again. Any idea's?

Simon B
December 2012