Halstead Ace High heating constantly running?

I noticed last night that my heating was still on, even though it should have gone off hours ago. When I went to the combi I noticed that the Hot water light was on and the combi was running, but no-one was running any water in the house. I have spent this morning trying to get it to stop, but the only way I can stop it is by swithcing it completely off. Even if I switch it to Hot water only and have the heating timer flicked over to 0, the Hot water light comes on and the radiators get hot. Any ideas as to why it would be doing this?