Problem with gas central heating, pilot light on and can hear the boiler firing but not hot.

Hi.

I have got a problem with my boiler. The pilot light is lit, and when i run the tap i can hear the system 'firing up' but the radiators are not warm and no matte rhow long i leave it, the water does not get hot.

Does anyone have any idea why this might be. It is quite an old system. i ahve been told that is porbably the central heating pump, but i have no idea where this is, where to find it or how to check if it is that which i causing the problem. Has anyone else had a similar problem, and if so what did u do?
If you called out an engineer, was it an expensive repair, as i have very little spare money.

Thanks.

It depends on what boiler you have, I think.