Horstmann H37XL Channelplus Electronic Central Heating Programmer Series 2 - 3 channel, 7 day

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Look at the wiring diagram for your make/mode of room thermostat that you are removing. If you don't have the instructions for it, a diagram is usually printed inside the lid. This will show which are the two switching wires, and which is the neutral wire. (If there is a neutral present. Not all thermostats need or have one fitted) unfortunately you can't rely on the wire colours. This will also be the case with the "similar Horstmann programmer" in your last post. The reason common terminals are provided outside of the unit, rather than connected to 230v inside, is so that they can be used to switch other voltages (some boilers use 24v) if required] Once the secondary circulation is out of the picture, the Zone 1* connections would be wired to the Central heating terminals of the Heatlink and Zone 2* connections to the Hot Water terminals. You should identify what each of the wires connected to the Horstmann does from the diagram you have posted, and then move them to the terminal that has the same function at the Heatlink. Are you saying that with the heating off the the heat satisfied also has a voltage, in the way terminal 3 has on the Horstmann. Of course the Nest will not only switch off when the time goes off, but also switch off when the correct temperature is reached, but the wiring doesn't know that, as far as it is concerned it's just going off.

Unfortunately, I don't know anything about the valve having never seen your particular type. But electrically speaking, the Channelplus and the Hive are just switches and are identical in their operation, so if your heating works with one, then there is no reason why it wouldn't work with the other.

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I would be tempted to try and trace it to see what it's connected to, for my own curiosity as much as anything, or disconnect it and see if anything stops working. Finally on the programmer what would be the point in providing term 3 to be live when the heating was off.

The reason I say this is because I connected the blue wire to this as your original instruction and the Nest never worked. I wondered if this only went live when the desired temperature was reached and it received a signal from the thermostat? By pulling outward and generally downward I can clearly see the single wall box because I am dragging that towards me too. It is not separating from the main unit or the stand-off.I'm not sure I understand what you mean by the above, because the switching elements of the Channelplus 37xl and the Heatlink work in exactly the same manner. A piece of plastic which may act as some sort of stand-off or interface - it's the piece that has the 2 screws in the top (not any more though). I suspect this is integral to the main unit - I only mention it because it is a separate moulding