Boiler won\\\'t light till heated up

Hi,

I have a 'traditional', not combi GlowWorm boiler (not sure of model number, sorry) and have a strange problem with the ignition of it. When switched on from cold (or via the Honeywell timer) the boiler comes to life (fan starts, etc) but no 'ticking' is heard trying to ignite. I can switch the boiler on and off, but the same happens. However, if i leave it 'on' for about 15mins, switch it off then back on again, the ticking starts, and the boiler ignites perfectly. When heated up the boiler turns its self on and off via the thermostat perfectly, but the problem comes back again when it cools down. Any ideas?

Thanks.

air pressure switch or the fan/pipes need cleaning

Thanks, Kevplumb. Is the air pressure switch an expensive thing to change? Also, is the pipe cleaning something I could do myself, or at least check them to find out if they need cleaning? I have no idea about pumbing, as you may of guessed!

I really apreciate your help with this, thanks.