Got a fully pumped system, with cylinder thermostat and 3 way valve (no
room therm)

HW seems to work OK - Heats up, cylinder themostat cuts off and boiler/pump
switch off.

However have got problem with valve (or rather the way it has been wiring!)

When HW & CH are selected (programmer only allows HW only or CH&HW) valve
remains in B position (HW only), boiler and pump power up and tank heats.
When the tank reaches temp the thermostat switches off.

At this point the motor powers up and moves to the AB position, the
microswitch is activated and the motor stalls in the AB position, The boiler
and pump however dont run and so the heating is in the off state.

If I manually slide the lever on the 3way valve over to the A position
(heating) then the boiler and pump switch on again

Which wires are connected incorrectly?

Any suggestions?