The thermocouple is the electrical sensor which detects the presence of a flame and allows gas to flow into the system, however they are prone to wearing and becoming unreliable. Rather than them letting gas through continuously, once broken the thermocouple won’t let gas through at all meaning your boiler won’t provide any heat. An estimate on price could be anything from £40-60 for the supply and fitting.
On more modern boilers the malfunction of a flue fan is a fairly common complaint. The flue fan is what expels potentially harmful gasses out of your property, and if it malfunctions, your boiler will simply refuse to start. Pricing estimates again can vary from anything between £150-250 depending on cost of parts and boiler type.
This is normally a sign of the motor pump failing to work and results in no hot water being pumped around your heating system. On occasion the top radiators may warm slightly due to convection heating whilst the ground floor radiators stay cold.