Balancing, more often than anything else. Water takes the easiest route, so the radiators nearest the boiler get the flow and the far end of the circuit gets what is left. Nothing is faulty; the lockshield valves have simply never been set. After that: circulation, where sludge or an undersized pump means the heat is being made and not moved. Then controls, where a thermostat in a draught or a timer nobody has reset is running the house to the wrong temperature at the wrong hours.
Heating is priced after the job has been seen, anywhere in the areas we cover. Send the postcode and a few lines about what it is doing when you contact us, and the answer comes back in writing.