Also on your prior ST699 connections. The hard link from L to 5 means that when the hot water was "off" power would have gone across to 3 (CH-On) You had a hard wired link between Live, 5 (common for CH) and 6 (Hot water on). That's why I thought your hot water was permanently on. There was also a hard link between 3 (CH on) and 7 (HW off). Then there were connections to all four terminals for CH on/off and HW on/off which is not normal. So I wonder if the other end of those wires go to the expected places. 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?). My question is how difficult would it be to replace just the timer and thermostat (with a wireless one) without touching anything else in the system? Would I need to replace it with a Honeywell model or are all the wires like for like? Item arrived prompty: I was informed of my delivery two days in advance and even had a phone call when the driver arrived- great customer service.

Sounds like there would be a bit more work involved in sorting the flexibility side out, as well as updating thermostatic room control, so I think I might just leave that bit alone until the day comes that I install a whole new combi-system.

Why settle for old technology, when Google Nest can do it all?

You can have hot water only, but you can't have heating only - the hot water has to be on for the heating to work, although I have no idea how the two are connected from a system point of view... Does that mean hot water tank in airing cupboard which is heated via the boiler? (You may also have a separate immersion switch for the tank). 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. Is it a DIY job or something a professional should do, baring in mind I don't want to replace the whole system, just the timer/thermostat of the existing system? It's all connected and I've set the time and date, all is okay. However, even though the lights come on, it won't fire up either the heating or the water!

Can you please provide a picture of the ST9400 wiring. Do you also have any pictures of the prior ST699 wiring to compare. If you don't have a picture of the ST699 wiring can you carefully outline the connections on the ST699 (terminal, wire colour, etc) and to which terminals they were moved for the ST9400. You said "I have cables linking:" did you mean "I HAD cables linking.. on the ST699".Prior to the ST699 failing did everything work as you would expect. What you programmed as the hot water on/off cycles is what happened. Likewise for the central heating. I do know that there is a Grundfos pump in the airing cupboard next to the cylinder and two white boxes connected to the pipes which I believe are motorised valves, but that is the extent of my knowledge - the airing cupboard is a bit of a spaghetti junction or pipes and wheel valves, and I don't really know what any of them do! I just opened them all when I first moved in and hoped for the best, I've never touched them since lol.