How can i fix my combi boiler, Worcester 28CDi?

I have my boiler temperature set between 3 & 4 for switching on first thing in the morning.After about two minutes i get a horrible banging noise in the boiler just like water hammer. This lasts for approx. 30 secs.then stops. After 30 mins. i can turn the temperature up to between 3 & 4 and there is no problem. To get over this problem if i turn the temperature setting to 2 for start up there is no banging. The boiler is approx. 6 years old. I asked the installer for advice who thought the main heat exchanger was partially blocked. I am not convinced because i get good heating and hot water.