CH Pumping Over - Control / Wiring Problem?

I've been having a problem with my Heating and Hot Water System that I hope you can help me with.
The system is a Fully Pumped system using the Y Plan wiring and a mid position valve.
The problem manifests itself as pumping over, usually early in the morning, but also at night. The problem appears to be when the DHW reaches the temperature set on the tank stat. Instead of switching off the boiler it carries on and the pumping over happens.
I managed to get into the airing cupboard today when this was happening. I noticed that both channels from the Programmer for Hot Water On and Hot Water off were live when tested with a neon screw driver. Being a bit more scientific I got the Volt Meter on it. The Hot Water on had 240V (or there about) whilst Hot Water Off had about 150V. The grey wire on the mid position valve also had a bout 150V on it (I think these two are linked?)
I had change the motorised head on the mid position valve suspecting this was the culprit last weekend, this made no difference. The previous control head was quite new as I'd had a problem of only getting heating with DHW a while back (in fact twice which suggests there may be another underlying problem).
I'm not sure what else to try short of rewiring the control centre, but I'm not sure that it would help. For reference I have a Honeywell Programmer, Programmable Room Stat and Tank Stat (all of which are quite new).
Any ideas would be appreciated.
Thanks Marc
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