On my Baxi 105E instant combi boiler the pushrod in the DHW differential stays out after the tap is turned off and hence the system stays in DHW mode.

I've cleaned and shined and greased the pushrod and cleaned out the inside of the diffential including the rod on the thrust plate.  I've checked  the diaphram - looks OK.  I've checked that the sensing pipes are clear and I've cleaned both DHW filters.  But no joy

I've checked the microswitch operation and its fine so it's down to the diff or whatever drives it.  

What should I try next.