Mexico super RS55- heating and water problems?

We have a back boiler( ideal mexico super RS55.). Before we could get central heating and hot water both independent of each other. Now we can only get heating for a short period less then an hour. Both heating and water needs to be switched onto together and the water becomes extremely hot in half an hour or so. It feels like it is going to explode.
Getting hot water on its own is also a problem and it overheats and gets too hot too fast.
what can be done to solve this problem?

Thanks