Removal *should* be simply the removal of the screws and a firm pull of the unit to unplug it from the baseplate, however even pulling with all my 100Kg, I'm not shifting it. Tiara, two channel programmer with 16/10 programme setting for fully pumped or gravity hot water/pumped CH systems.Select ON/OFF twice a day, ON all day,or permanently ON or OFF with advance switch and 9 pin wall fixing plate. So there you go. It's given me some confidence to look at the Heatmiser wifi-connected thermostats next.

Hey guys, it seems like you are all very helpful, and as I know very little about central heating/boiler systems (I still can't figure out whether I have a "Fully Pumped" or "Gravity Fed" system), I thought I would ask for help. What I really want to do is to install a NEST in place of the two thermostats, and a single timer for the water but I don't yet understand what wiring my system employs and how I might convert that for NEST.

As for your easy to understand explanation about my system, that was great, I'm a lot happier now I understand what is actually happening in my home.

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. 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 Although it says "not user serviceable" I thought I'd have a looksee anyway - I'm not without some skills in the repair and replacement of PCB components - however I can't get the bloody thing off the wall. There's a baseplate which is fixed to the wall/hole in the wall onto which the controller should plug, and is secured with two screws on the top.This would not allow your system to work properly on the CH, so I have to assume you have already 'had a go'.