That's really hard to say leighton without knowing the type of boiler or the 'symptoms'. Could be the ignition, could be that the thermostats are faulty, could be that no gas (I'm assuming it's gas although could be oil?) or water is getting through to it.
You're best off calling out a corgi-registered engineer because if you or anyone unqualified touches it and something goes wrong your house insurance is void and more importantly it could be dangerous!