The original question was a good one and I'd love to have heard a decent answer. It's taken me hours to discover the answer to my heating behaving the same way. I finally found JustAnswer.com and chatted on-line to a plumber which cost me £11. The answer for me is that the thermostat controls a tap that decides between the tank and the radiators and that tap or valve was a bit stuck. You'll find it between the hot water tank and the central heating pump if your system is anything like mine. It's just in front of my hotwater tank. There's a lever on the side of the little silver 50x100mm box on the top of the valve that is stiff to push when the boiler is heating the tank and kinda free feeling when it's in radiators mode. I think just moving the lever across and back freed it so now when I turn the heating and hot water on the heating goes off when it gets to the thermostat set heat. It goes off by turning the boiler off if the boiler's not needed for the hot water but it goes off by turning the valve to the radiators off if the boiler is needed for the hot water. Make sense? It might be going to need replacing one-day but for now it's working fine for little more than having investigated and worked it manually. Thanks to JustAnswer.com not to the reply given to this guy who asked a very sensible question on diynot.com Maybe if he'd had this answer six years ago or whenever he had the problem it might have saved him some trouble. I hope it helps you today. Sincerely and in good faith. TEUC
The difference was that you made a phone call where incidentally the guy on the other end was being paid for talking to you. This more time efficient form of communication enables him to quickly gather your level of understanding and respond appropriately. On here we do it for no money and through written words it is harder to gain that same rapport with those who post here.