Hi, before I ask the question, just to let you know I am a qualified electrician and am only doing the electrics part of the system. My boiler is Myson floor standing lump-of-cast-iron boiler approx 20 years old. Model number is Myson Marathon 800b. It originally had an internal mechanical timer. This was disconnected a few years back and replaced with a Lifestyle programmer by our local (trusted) heating engineer, and has been working OK for a while. We got it serviced recently, and asked them to look at the three port valve as it was only supplying hot water, and you had to move the valve to manual to get heating. Unfortunately, because the thing is in a difficult to get to place the guy couldn't do anything with it at the time, so it was left that we'd use manual until I could dismantle the cupboard and rebook the call. I dismantled the cupboard and took it into my own hands to replace the head of the valve myself as I have done it before. I took the front off the boiler to wire in the thing, and noticed all of the wiring was brittle and the chock strip connectors were all very brittle and fell apart. I decided to get a proper wiring centre and relocate all of the cables to that, leaving just the switched supply to the boiler only in it. I did that last night, and turned it on. As far as I can see, the electrics are working ok, but after a short while there was a noise in the loft, and the heating was boiling up the overflow into the header tank. Turned it off PDQ! Where I THINK I might have gone wrong is that the boiler has a thermostat on it. I couldn't find a wiring diagram for it online, so decided to leave it out. I know - dumb! i'm guessing that without the stat shutting the boiler off when it got to temperature allowed the water in the system to boil. I think reconnecting the stat will probably be the answer. Would I be right in assuming that to connect this stat, all that is required will be to have the switched live coming from the wiring centre on one terminal, and the other terminal of the stat to the live to the boiler? I.E. place it in series between the switched feed to the boiler and the boiler itself. It's a mechanical dial that has numbers 1 - 8 and off, with a thickish wire running into the boiler cast iron lump. Also, a second unrelated question. The old wiring of the controller didn't have a cable from the hot water off terminal of the controller to the tank stat / valve. It used L and N plus HW and heating on only. I guess the tank stat provides the hot water off signal to the valve. Is there a reason they might not have used this cable? I have wired it in as I followed the honeywell y plan diagram. Many thanks!