I am wondering if the stand-off comes away from the main unit. I can pull the main unit away from the stand-off enough to get a screwdriver into the gap but it's not coming away consistently - only where I exert pressure so much that the plastic deforms. 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. Insert a wire link between the terminals where the two switching wires have just been removed from, so that they are now electrically joined together.

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. 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. I am assuming here that you are referring to a 'Secondary Hot Water Circulator' that circulates the domestic hot water around a ring main so that hot water is available instantly at each outlet, then the answer is yes you will need to move that to another timer. This is the first time I have come across this in a domestic property, but can understand why if you have an unusually long pipe run.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. I'm wondering this due to the wiring indication that came with the valve, and also the number of wires in the connecting box. If this is indeed the case, I don't think I can use the Nest unless I use some sort of relay, or change the valve.

I have only ever once come across a system with a 'central heating off' connection that contained a very uncommon type of motorised valve (MoMo, or Motor on, Motor off) that requires this connection to close it. For instance, I *hope* the cabling to each thermostat allows for me to install the NEST power supply (supplies?) in the hot tank cupboard (where the timer currently lives) and power each thermostat (remotely) that way. 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?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] In reality, the blue wire may not be actually doing anything, but only the original installer would know. However as I said earlier, you are simply exchanging one electrical switch for another and the corresponding terminal at the Heatlink would be (1) Heating Satisfied. Finally on the programmer what would be the point in providing term 3 to be live when the heating was off. You will have to link the terminals L, and the two 'commons' 2 & 5 together at the Heatlink. [This link is made internally in the Horstmann]

Finally at least one radiator in the room where the Nest Thermostat is located should not be fitted with a TRV, otherwise it could interfere with the operation of the Nest and prevent it working properly.

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 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.

I can get a screwdriver between the wall box and the stand-off but there's no way they are going to separate without my breaking one of them.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. 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. Then, once the common at the Heatlink is connected, both devices will function identically to each other. I also don't really understand how each of the two-way valves operate - whether the timer "job shares" between the two zones and whether two NEST thermostats can replace that functionality. Or if each thermostat demands heat and the timer is on for each zone, both valves just open and they get served with hot water equally.