i've almost the exact same problem.
central heating on mine however works fine but the hot water would go through a cycle of first of all heat up and then cool down.
switch off mains power.close flow and return valves and cold water inlet.release water from system by turning the pressure relief valve a quarter turn.
remove the white coloured drum "domestic hot water" heat exchanger and give it a thorough clean by running hot water from a bath tap through the pipes.cover one of the pipes with your thumb and allow it to fill up.then cover that one as well and shake it around and when you let go youll see all the muck come out.repeat this until clear and reassemble.
this is only one of a number of reasons however.hope this helps.
araf
January 2009