vokera compact boiler?

My boiler broke down during the cold spot before xmas., A new return valve was put on, followed by a vermister on the heating. However, there are no lights displaying on the boiler although it is firing up fine. When the heating has been on for some time (and it is feeling v hot) if I try to use the hot water it turns the boiler off. When the boiler cools down, I can then use the hot water. It seems to be a case of overheating somewhere bit my plumber says I need a new boiler. I have been told it may be the divertor valve sticking. Any ideas please.