Vokera boiler works in heating mode but not Hot water only mode?

I have a problem with my boiler it is a Vokera 20-80 RS flowmatic. The flowswitch tries to move but does not operate far enough when just on hot water even though the 3 way diverter valve moves OK and the water is diverted through the DHW heat exchanger. If I allow the boiler to fire up in heating mode then switch on hot water the flowswitch remains activated and all works fine. I have checked the flowswitch and all seems well (replaced the diaphragm just for good measure). Pump seems OK it delivers plenty heating and hot water when operating. I have removed and flushed out the main heat exchanger and DHW heat exchanger and both seemed OK. I am left with either the pump head has deteriorated slighty and at the high flowrate in DHW mode it cannot give enough head to operate the flowswitch (??) or the flowswitch has some sticktion adding to the force needed to operate it (??). Could it be anything to do with the pressure balance bladder? I don't think so but I am asking?
Also my boiler does not have the auto-bypass arrangement at the back of the heating manifold. It is just a simple spring operating against a rubber diaphragm that works the flowswitch.