We have a Vaillant VCW GB 221 H boiler, which has developed a problem.The heatin

We have a Vaillant VCW GB 221 H which has developed a problem.The central heating will come on when I operate the hot water and the hot water sometimes gets hot sometimes not.The water section pin rises when I turn on a tap and the micro-switch operates firing the boiler the diverter valve mechanism moves left and operates both upper and lower micro-switches the flow to radiators heats up but not the flow from the heat exchanger to the diverter valve. I do not know how the servo valve effects the diverter valve and how to test this.The diverter valve always moves to the left weather I select heating or hot water ,this doesn't seem right to me as one of the micro-switches operated by this never goes open.I feel that the problem is either the servo or the diverter valve but don't want to buy either as their maybe some other explanation.expert advice would be greatly appreciated