Searching the other threads have found a couple of links that are very helpful, one with interactive diagrams and the other access to some downloads from the Honeywell website (you will need to register as a member to access downloads):
Honeywell Y-Plan Explanation +
Honeywell United Kingdom Heating Controls
Will also post this image too for reference:
Y Plan Wiring Diagram:
And I've also found the explanation below:
How the mid-position valve works
Port A = CH
Port B = HW
Port A+B (Mid-Position) = CH + HW
When the valve has no power applied, it's in the "rest" position with Port B (HW) open. In this situation power is applied via the Programmer HW ON, HW thermostat Common and Call terminals to the Boiler
If CH is required at the same time as HW, voltage is applied to the WHITE wire and the Valve motor runs via S1. When the Valve reaches Mid-Position, switch S1 moves over. The motor is now fed via S2, Resistor R1 and Diode D. The Diode rectifies the AC current so it becomes 'half wave'. The effect of this is to "stall" the motor in Mid-Position
Resistor R1 limits the current through the motor and consequently the residual magnetism. This ensures that, when CH goes off and 240v is no longer applied to the WHITE wire, the spring in the Valve is able to return the Valve to its original (Port B open - HW Only) position. The Boiler is still supplied via the HW thermostat as in the HW only position. The motor is also supplied via the ORANGE wire and Resistor R2 with a lower AC Voltage, this also prevents the motor becoming permanently magnetized and ensures the spring return will work
If HW is now satisfied or turned off, 240v is applied to the GREY wire, via either the programmer HW OFF or HW stat SAT terminal. This applies 240v to the motor via switch S1. This is sufficient to move the motor from Mid-Position to fully across (Port A - CH only) open. The Boiler is no longer supplied via HW ON and CALL terminals but, as the Valve reaches the end of its travel, switch S2 moves over. 240v is now applied via the WHITE wire, S2 and the ORANGE wire to the boiler
If CH is now satisfied, or turned off, the WHITE wire no longer carries 240v, so the Boiler goes off. The GREY wire, however still carries 240v, so the motor is held in the Port A position (CH Only). Switches S1 and S2 both remain in the "switched over" position. In this situation Resistor R2 now performs a second task, it reduces the voltage applied to the Boiler via the GREY wire to between 50-150V. This is lower than the Gas Valve or CH Pump requires to operate, so the Boiler does not light. The Valve will stay in the CH position until either HW is called or the power is turned off
