ProDave

That looks like a 1W switch (simple on /off) A 2 way switch would have another terminal in that hole circled in red. For the few p you save I only buy 2 way switches

How much did you use in your house? No more than 500M in mine, if that, and by buying new offcuts you can get it pretty cheap. e.g 90 metre "offcut" for £60 with postage https://www.ebay.co.uk/itm/316187121886? 5 of those would have done my entire house. At that price, just put it in, it gets used, or if not, you have not wasted much.

I used to build industrial control systems and there is a defined risk assessment that considers likelihood of a failure and consequence of a failure to determine what sort of control system was required. For most of what we did (machines that could kill if the safety systems failed) that was 2 contactors in series, with a safety relay monitoring the state of the contactors and would not allow the machine to start if it detected one of them had a stuck contact. Now given a boiling unvented cylinder could explode and kill someone if the safety systems failed, I would not want to be the one standing up in court explaining how I designed my own 24V contactor driven "safety" controls rather than using industry practice as described in the installation manual. I did encounter an ASHP installed to it's manufacturers instructions (I forget which make / model) and that connected to the immersion heater in the UVC WITHOUT a thermostat on the immersion heater itself. I was most uncomfortable with that, but I neither installed it or altered it, just observed what was fitted.

I am not familliar with the heating system you propose. Is it warm air heating delivered by the MVHR? If so previous discussions suggest to deliver enough heat it will have to run at much faster flow rates than normal mvhr. I would put those UFH pipes in and go that way. You will almost certainly only need that on the ground floor as we have. What do the heat loss calculations say is the heat input needed on the coldest winter day you can expect?

Does your heat pump expect to connect to the immersion heater? If so that dictates a lot of the controls and you have to work or adapt around that. In my case the ASHP does connect to the immersion heater (though I have disabled all use of that in the settings) and it does so with a control box supplied with the heat pump, which contains a contactor and mcb's. To integrate that with my PV dump controller, I modified the supplied box to single pole switching, so only switching immersion L via the contactor, and then connected a solid state relay in parallel with the (now single pole) immersion contactor switched from my dump controller. The use of a remote SSR for the dump controller was a big motivation for making my own. The immersion heater will have it's own normal thermostat and secondary over heat protection should that one fail. The cylinder mounted thermostat with a probe in a cylinder pocket is to protect the cylinder from over heating when fed from an external heat source, so should be wired to close the motorised valve on the cylinder input coil if the cylinder is overheating.

Okay 1 more. First self build, over 20 years ago now. Originally planned a square shower tray. UFH pipes laid out to suit that. Floor marked with big marker pen showing where every single pipe was. Changed plans. Decided on rectangular shower. Waste in a different place. Cutting the hole for the waste, a fountain of water started to come from under the floor. Yep I had failed to see my own markings and cut right through an UFH pipe.

Neither. For simple white switches, I don't think you will beat Click Mode for quality, value and reliability. But Screweys don't sell them. Toolstation do. https://www.toolstation.com/click-mode-10a-switch/p73508 A little "quirk" of these switches is the switch mechanism can be removed from the plate, enabling you to mix normal and intermediate switches on the one plate. Almost like a grid switch.

As above , if it is just a workshop / shed (i.e. you don't intend to fit a car in it) I would be very tempted to have two buildings within the size limits for permitted development and not requiring building regs. Given the aparent L shaped nature of the garden from your plan, I would personally tuck the building(s) just around the corner to the right so not obstructing the straight run of garden from the house.

Yes is does sound like it is not shutting off. Only way to find out for sure is remove it, then you might as well fit the new one. PITA having to drain down to do so.

^^^^ Ah yes safe zones. Everybody know never to drill a hole in a safe zone without checking first. Except the person hanging something on the wall, and then of course they measure meticulously to ensure the thing they are hanging exactly lines up with the centreline of the switch. BULLSEYE

These actuators are a pain to test as they don't work with the actuator off the valve, the mechanism relies on being on the spindle of the valve and just won't work properly if you try and energise it off the valve. I always find it is difficult to tell if water is flowing as you don't have a flow meter, you can only go by is the water hot both sides of the pipe etc, so when it closes you have a while to wait to see if the pipes cool down or not to confirm if it has closed and water has stopped passing.

You should be able to shut it off with little force. Does it turn freely with the head off? It will only turn roughly 90 degrees, and as you say the closed position should be a soft stop where the flow is plugged. If the plumber could not check that and passed the buck to an electrician, time to get another plumber. Is the valve seemingly opening and closing correctly with the head on? Can you see it rotating?

Better than that, but no photos. Drilling a 16mm hole through a thick stone wall for a cable. When it broke through on the inside, I cut back a bit of the plasterboard, to find the drill bit had emerged through the wall between two 22mm copper pipes that nobody knew were there. An inch either way and it would have been a wet problem to fix.

A previous job, one of the engineers never seemed that bright to many of us, and nobody was really sure what he was doing. Then after a while he was sacked and marched off the premises immediately. It turned out he was spending most of his time running his own business.

Thank you for coming back and reporting the outcome.