Mike

As per my post above, yes.

Exposure extent, rain quantity, rain duration, wind speed, brick permeability, quality of the mortar joints, low height of the wall...

Some of the work of a company I worked for was fixing NHBC insurance claims. One of them was retrofitting cavity trays that had been omitted by the original builder; stage 1 of that was cutting out bricks to verify the cause and identify the extent of the problem, so I've seen myself just how much can get through - a lot. Never omit cavity trays.

Very likely, unless the brickwork is rendered. In an exposed elevation it can run down the inside of the external leaf pretty freely, especially if the joints aren't well filled.

Yes, from memory I think my Elemex model has a 7" stat too, so temperatures in the top half will have no impact. It will switch off either if the temperature builds up lower down (the hot water isn't being pumped away quickly enough), or when the incoming water reaches the stat temperature.

Makita SP6000 Dewalt D2513 as a good compromise between power and weight You may want to go up the range, but I've got the small Festool CTL SYS + DIY dust separator

I have a mixing valve too, but only to prevent high temperature water entering the underfloor pipes in case someone mistakenly sets the Willis thermostat too high. It's therefore set to 50°C, not the target flow temperature. But it will still mix the water temperature down a few °C as mentioned by @JohnMo. IR image below. With the Willis thermostat set to about 30°C the water coming out of the Willis at 45°C-ish and entering the floor is about 35°C. Other temperatures are misleading as some of the pipes are insulated.

Thanks for the added info; I've lightened your photo which helps too see what's going on too. So part of the problem is pushing the clear polythene, or maybe the black plastic marked 'A' (or maybe you've tried both) both through the gap where I've added the arrow? Bearing in mind that the minimum overlap is 100mm, you don't need to get a large piece of DPM through that gap - an offcut of 300 x 200 - ideally a little more - would be enough, if it overlaps the DPCs by 100mm. If the partition is too tight to the external wall, then I'd carefully (without damaging the existing DPCs) take a few mm off the timber using a multitool with a long blade. You could double up the vertical timber if there is any question over the somewhat reduced strength. Having said that, if the timbers on that that internal partition are 100mm wide, it looks like the DPC beneath doesn't project enough to provide the minimum 100mm overlap? If that's the case then you will need to get a new wider DPC underneath its full length. On the external wall, I'd have preferred the DPC to have been turned up but, if it's turned down and still overlaps the DPC & other DPMs by 100mm then I'd find that acceptable, but you'd need to verify that your BCO & any 3rd party certifying insurer that may be involved takes the same view.

For the second winter in a row, my Wilis UFCH is operating manually, but by next winter it will be more sophisticated. A Shelly Pro 4PM will be capable of scheduling the days and times to switch on and off & providing remote control. Upstream of the Shelly there will eventually be a Raspberry Pi which will calculate the heat required, monitor temperatures and command the Shelly (which will become a fall-back in case the Pi is out of action). Either way, the Shelly will send control signals to a pair of 25A contactors (one for each* Willis heater) and will directly control the pump (since that draws little current). A Hager 60060 load-shedder will temporarily cut the contactor control signals if the mains supply is under heavy load, but in your case, @Antonb182, that would be the place to put a thermostat. However, note that with UFCH isn't as reactive as radiators - there's a significant time lag between heating the floor and the air reaching the set temperature that you'd need to take into account to avoid overheating. Schema + sketch below. All the this (excluding the Pi) will take up one row of my consumer unit. D4.4 & D4.5 in the schema = D21 & S1 in the sketch; not got round to updating that... *Although I could heat my place with 1 Willis heater, I've chosen to install 2. That's partly to have the option to use both to load the floor with heat during off-peak hours, but mainly so that I can switch to the second if the first should fail.

I'm not clear on exactly what you mean - or rather that at a junction of an internal wall and external wall I can't envisage a problem, which probably means I've not understood you question. However any DPM / DPC is likely to be too thick to fold into hospital corners. There are pre-formed DPC corners on the market, for a price, that may help. But the fundamental rules are that all DPM and DPC junctions must overlap by at least 100mm and be to the satisfaction of your BCO.

I agree that it looks like water splashing, mostly below DPC level so nothing to worry about. No harm adding drainage channels - it should reduce the splashing. They're just common bricks, intended to remain invisible below the ground level. The mortar was almost certainly never there in the first place, though if a tidier brickie had done it then it would be.

Maybe cladding + additional genuine Compriband between the cladding and the window?

That's an interesting question. Just went hunting and came up with this chart, showing public approval & disapproval of Government between 1957 & 1991. The only periods when more people approved of the Government than disapproved, were 1957 to 1962 (Harold Macmillan, Conservative, Keynesian economics, prosperity, "You’ve never had it so good" - until the Profumo scandal), and 1964 to 1966 (Harold Wilson, Labour, low unemployment, economic prosperity, major social reforms, "white heat of technology" - until defeated at the polls). Source: Social and Political Change in Britain (1945-1991) | ROPER CENTER

The advantage of moisture permeability is that, should any moisture get into the structure, it has an easy escape route - for example to reduce the risk of rot in timber windows. That's why I chose to use it on my current project - though I had to rip the cheap stuff out of the installers hands and give them the genuine product!

It's fine for sealing membranes to each other or to adjacent surfaces - I've used it myself - but I wouldn't use it to seal plywood joints. If you're sealing ply for airtightness then I'd use an airtightness tape. BTW it does set, but it will remain flexible - to a point - and tacky.