Heating won't switch off?

Okay, it's a long one....

We have a heating only boiler with a separate water tank for the hot water. A while back the motorised valve was faulty and we were getting the heating coming on with the hot water. We got someone out and he replaced the body of the valve. A month later the boiler packed up and we had this replaced about two weeks ago. After the boiler was replaced we were getting the same original problem - the heating coming on with hot water, and vice versa. We called out the same company and were told that the head of the valve needed to be replaced. That's now been done but here's the problem. When I tried to switch off the heating only the boiler stayed on and the thermostat stayed on. So I thought I would turn down the thermostat to zero. Only the radiators stayed on and were so hot I could hardly touch them. Next day I tried again to turn off the heating. This time the thermostat went off but the boiler stayed on, as did the radiators. So the only way I can put the heating off is to switch the boiler itself off. Water is also scalding hot. Any ideas? Much appreciated - thanks.