I have a truly ancient Potterton Kingfisher CF55, probably about 35 years old. It has generally been as good as gold, but we are planning to replace it in about 18 months time.
For 6 months we've noticed that the boiler case gets very hot. About 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 20 seconds, then on for 40, off for 20 etc.
I've had a heating engineer round and he thinks it's either 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. 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.
My gut feel is that it's the pump, as the thermostat seems to be doing what it should, ie cutting out when it gets too hot. I think the heating engineer's advice makes sense, but if it is more likely it's the pump - I don't want to waste £60 on the thermostat.