I was expecting the ‘twice’ indicator to illuminate whenever I selected ‘twice’ - of course the light only illuminates during the two periods programmed. The instructions for the hive does not say how to put it back to normal operation so assume that its just the same method as in gravity feed. If so, these will be from the motorised valves and would be connected to the internal switches I mentioned earlier. From an electrical point of view they are the same, so you are simply exchanging one switch for another identical one. However, the ST699 has two simple switches inside, one for heating and one for hot water, and so does the Hive.

If Nest Protect detects carbon monoxide, the Nest Thermostat can turn off your boiler, because it's a common source of CO leaks. The use of three blue cores, two brown cores and one red core coming into the ST699 is a recipe for confusion. I just opened them all when I first moved in and hoped for the best, I've never touched them since lol. Does the timer allow you to have HW only in the summer but, in the winter you must have both CH and HW on at the same time?

Both the valves, the pump, and the cylinder thermostat have wires coming off them going into another white junction box which then has only 3 wires coming out the other end going underneath the airing cupboard (presumably to the timer downstairs?

If you have a S-Plan system with a single heating zone 3 for HW-On and 4 for CH-On, nothing to either 1 or 2. I do have hot water tank in airing cupboard and I've just been and looked and there's nothing in there apart from pipework and the switch (we have another switch in the kitchen which I assume is linked to this). If with your current system, you can have the only the heating 'on' and the hot water doesn't come 'on' as well, then you won't have to change the setting of the Hive. They've both got what looks like a switch with A - B settings, the one on the pipe running down past the cylinder is set to B, and the one on the pipe running into the cylinder is set to A, someone has also written on this one with a marker pen putting a C next to the A and an O next to the B (presumably meaning open and closed?The programmer is suitable for surface or switch box mounting and comes complete with a built-in battery back-up. Then when I switched the main switch on, the pump ran for a second or two then stopped, which sounds to be what should happen. The old room thermostat can't simply be disconnected as this would leave the heating wiring 'open circuit' and the heating will not operate, so, the two live wires that used to go to the thermostat then need to be joined together to complete the circuit. Any help would be appreciated I can’t see where my wiring is wrong but as you may have gathered this isn’t my strong suit. This has lead me to think that while I'm replacing valves anyway I may as well use the opportunity to fit Thermostatic valves to all the upstairs radiators.

First apologies - I didn't realise; as Sam Spade says, Y plan doesn't have a switch in the diverter valve for HW. As you have a 3-port motorised valve connected [the clue is that there is a wire connected to the HW off terminal] then inside the motorised valve is a microswitch that controls the boiler when the heating is required, which means the Hive won't be controlling the boiler directly. As an aside my 'bay' window is more like a box on the side of the house, it's not full height and as far as I can see the large window sill is the only thing separating it from the outside, which seems like a horrible design from an energy saving/warmth point of view considering all the other walls have cavity wall insulation.However, what is of direct interest to me - and you struck on this yourself, Graham - is that the programmer in CH mode supplies the motorised valve via the room stat. Honeywell Home ST699 1 Day Classic Programmer is an easy to use replacement programmer on gravity primary or fully pumped systems. It's often the case that a standard cable L, N, E is used to connect a room thermostat and the N conductor is often used as the switched live.