cannot control hot water vaillant ecoTEC plus 624?

We have a three year old ECOtek plus 624 that has been fine until now. We have just installed a new show with pump and mixer valve. The water temperature is not controllable it is either scalding in the shower, and hot at the taps or cold at the taps and lukewarm in the shower. Small adjustments to the bioler controls make large changes in temperature at the shower. Hot water does not seem to funsction at all unless the central heating control on the boiler is on.

We have replaced the shower mixer but the problem is still there.

Any advice?