Hot water works but not very hot. Heating comes on but when warm goes off??

Hot water works but not very hot. Heating comes on but when the radiators are warm the burner cuts out and radiators go cool. The place never even gets the chance to warm up??

If the system is left to run the radiators never get hot and the boiler
fires up for about 2-20 seconds and fires down again. The system is also burning a lot of fuel. Eg £20 in two days.