Done that and the valve moves freely, but not very far. Not sure if this is normal or not (see above), so I'm off to the local plumbing supplies to inspect theirs. Maybe buy a new one.
I'm being told elsewhere (diynot.com) that it's my thermostat, as that's what instructs the boiler not the 3 way. Can't see how draining down etc could affect the thermostat though....
Far be it from me to cast aspersions on another forum user (but I will anyway), but the DIYnot advice sounds like bobbins. The thermostat is a simple beast, when heating or hot water is "on" (be it by timer or always on) it checks the actual water temp vs target temp and calls the boiler to fire (if required), triggers the pump to spin and triggers the change in the relevant diverter valve. The diverter valve then goes from closed to open and sends a signal back to say its done it. I *believe* that the thermostat looks for this signal, and will lock out your system as a safety measure if it doesn't get the response its expecting. Hence why there are a number of wires to the valve head, otherwise all that would be required are +ve and -ve and ground.
Your boiler fires and power is getting to the valve head. Sounds pretty much like its doing its job to me...If the motor spins but the arm doesn't move, then that's your culprit.
Personally I don't think resistance in the valve is a factor, but you could test this by unscrewing the valve head from the body then setting the hot water going. If you get the same result (ie motor spins for a bit, the arm on the valve head doesn't move then the boiler locks out), which I bet you will, then you've eliminated the valve body from your enquiries.
I've only installed pipework for combi systems up to now. So I only have experience with the 1 in 1 out type valves that are used on central heating systems with multiple zones. But they work on an identical principle and the amount of articulation you are getting from the valve head sounds about right to me.