what is wrong with my Vaillant Sonnet 18?

The hot water is working fine but the heating system is not working.
Can you tell me why, as it is a very old but reliable boiler and we never had any problems except British Gas keep telling us to replace it due to it's age....which we are aware of but it has been very reliable all those years and we are planning on replacing it this summer but we fill the problem might be something simple as we are getting hot water.