Worcester 28i Junior Banging when CH fires up and scale in hot water?

Our boiler is just over two years old and has been playing up for nearly a year. The hot water is fine, but when the central heating fires up the boiler bangs and gurgles, very loudly. When the CH is on black scale comes out from the hot water taps. It has actually stopped working previously and we have had the system flushed out a few times. Please could anyone give me an idea as to what is causing this as it has cost me a fortune so far and is ruining my new kitchen and bathroom fittings as the scale is so bad that it has blocked gauze filters on the taps and I cant get the scale out even when I take the filters off.

Thanks

H