Potterton performa 28i - heating won't come back on?

If my performa 28i is in heating/hot water mode and the hot water is used for any length of time ie, having a shower, the heating will not come back on. The temperature neons are all lit but no faults are showing. The only way I can get the heating back on is to turn the boiler off and run the hot tap until cold. If when I turn the boiler back on the temp neons have dropped to 60 deg's the boiler will go back to heating mode and will work ok until the hot water is used again.