Central Heating - Problem with 3 way valve wiring.

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?
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Have a look at http://content.honeywell.com/uk/homes/systems.htm and scroll down to the wiring diagram for a Y-plan system.
Yours should be like this, except for replacing the room stat with a link between 4 and 5 in the 10-way junction box.
The most likely candidates are either a missing live feed for Hot Water OFF from the programmer to terminal 7, or one of the connections to the cylinder stat missing. The grey wire to the 3-position valve *must* have a live feed when hot water is off or satisfied - otherwise the whole thing won't work.
Roger
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