Worcester boiler 35CDi?

What can be wrong with my boiler? I first noticed a problem when my shower began running hot then cold, hot then cold every minute or 2. There seemed to be some banging noises and steam expelled from the sides of the boiler. This then stopped for a while and returned to normal. Then a week or so later we noticed that the bath wouldnt come up to temperature either and we were having to run the hot tap slowly to get any warm water never mind hot. Now another week on and i cant even get hot water at my sinks in the house- it only reaches luke warm everywhere now. The central heating is absolutely fine. Where do I even start with finding the solution?