The reason why the spindle retracts for CH is that there is plenty back pressure in the diverter valve to create excess pressure in the bundy pipe that feeds the flow manifold. Result is the spindle gets dragged in as soon as you run the boiler for central heating.
During HW, primary water flows through the white vessel, through the pipe behind the gas valve to left hand manifold to feed the pump. Now you have a virtual short circuit in that loop resulting in minimal pressure created in the bundy pipe. So, for HW with no ch demand, spindle may move slightly.
Nothing to do with the diverter valve, more a case of trapped air (have you vented the white vessel) or weak pump (make sure it is on speed three)
Another issue that even I have been caught out (I repair/ overhaul plenty of these beasts) is the daiphragm you have replaced. There is a batch that is made from thick rubber that is not very resiliant. this diaphragm will play havock with the flow manifold. get a proper vokera one if yours is a copy part.
The pie at 12.00 o'clock position on diverter. Place a 1/2" washer in 3/4" washer hole. Fit that on the 12.00 o'clock union on the diverter to create extra backpressure
