Having said that I suppose it can't hurt to put a TRV on the kitchen radiator as that doesn't really need to be on while we're in there cooking, worth considering I suppose...

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? 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. I always thought I would need to replace the boiler system to get these but (unless I'm very mistaken) it's just a case of swapping the valves on the existing system. Not so worried about downstairs but our upstairs rooms are very prone to overheating and I think TRVs would really help and save money. 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. Whilst wire colours do necessarily mean anything on a central heating system, the combinations you have are a bit unusual. For example blue being used for CH on & HW on and brown for hot water off. That's based on your indication above about wiring on ST699. 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).Yep like that - they're called 'Lifestyle 2-port motorised valve', there is one underneath the pump on a pipe that doesn't go into the cylinder, and one on a pipe that come off the other pipe and goes into the cylinder about halfway up it. 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! 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. Apologies for the focus in these pictures, I can assure you it isn't down to your drunken eyesight. I don't know what my digital camera was playing at today (I really ought to read the manual). Step 5. Here's my shiny new cell, coloured a rather fetching green. As you can see, it has PCB mounts but that's okay, it'll still fit.

But this has got me thinking, why stop there? Our boiler is controlled by an ancient looking Honeywell timer which from Googling I think is a Honeywell ST699 (except our version features a delightful brown/beige colour scheme!) There is also an equally old looking thermostat dial in the hall (just above the radiator which doesn't strike me as an ideal location!).

Is it as simple as looking at a wiring diagram for the ST699 and attaching to the corresponding connection of a new timer? Step 7. If all is well, the display should now be active even though the unit is not being powered by the mains. 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)