I have an ancient Potterton Kingfisher CF55, about 35 years old. It has generally been as good as gold, but we will replace it in about 18 months.

For 6 months we've noticed that the boiler case gets very hot. 4 weeks ago, the boiler started short cycling, ie it comes on for about 5 or 10 minutes initially and then switches off for about 15 seconds, then on for 40, off for 15 etc. It's the same with heating & hot water.

I've had a heating engineer round and he thinks it's the pump or the boiler thermostat. He said that the pump looks & sounds like it's working, but he can't be sure.

I can either replace the thermostat or the pump. While he thinks the pump is slightly more likely, he advised trying the thermostat first on the grounds that it will cost about 25% of the pump replacement, so it's a better bet.