How can i fix my Potterton Heating system/Boiler Mate II?

I have a heating system that supplies constant hot water (and heating), (it is not a combi boilder). My hot water has now started to become luke warm or cold at times and hot at other times. Can you tell me why this is, it has never done it before (it has always supplied piping hot water to all areas, sink, shower etc). My heating hasn't been affected.

Potterton Heating Syetem / Boiler Mate II