Miek

That's a crossflow design I'd say. And therfore less efficient. As @RobLe pointed out you really want to start with a counterflow design. It makes a very big difference to efficiency.

The hot air gun helps the tape stretch, flex and stick better

I did use a roller a bit but found a roller is not great if the thing your rolling against is rough, like blocks are.

The reason you've been given the job is its fidly to do right. I folded inside corners and taped outside corners and it took ages. A Hot air gun is your friend

One of my favourite Welsh phrases.. 'He's on his own' 🤣

I wouldn't worry. Pack it out with batten if you feel the need

Fill the void with insulation, 'foamed in' PIR would be my choice.

I use a hand saw. Put the sheet on saw horses with a couple of scaffold boards to fully support the sheet underneath, then neel on top of the sheet and cut along using a straightedge as a saw guide. I find this more accurate than following a line. Oversize by 1 or 2mm when cutting for a tight fit .

I think this is probably your best solution to heat the whole tank. Also has no moving parts so no pump needed. Often used to retrofit solar thermal heating to an old tank, so it's a proven solution. As with anything relying on convection use big bore pipes and swept bends to reduce pipe friction hindering the circulation

I've done this on my own too. Put the boards on trestles on the ground floor , vertically, so you can reach down and grab them from above. Ball ache but quick really.

If you had to move the car to move the bins up the sump plug version that would be annoying.

Not sure what your doing to generate that level of heat! More like 100W per adult.

The Mixergy has dedicated tappings for the PHE circulator, unfortunately in a retrofit situation the use of existing cylinder tapings is mandatory. You can add your own tappings believe it or not, provided you have an immersion element boss. Buy a tank connector, Cut a hole where you want the tapping, drop a string in from the immersion boss and hook it with wire in to the new hole, drop the tank connector down the string so it locates in the hole , screw on nut. Pat yourself on the back :). I've done this a few times on old tanks. You can even retrofit secondary coils but that can be quite tricky.

Give it to the farmer, make a friend with someone with useful stuff, might come In handy when you need a telehandler later in the build

Better to cut that cold if its coated in paint zinc or plastic . Make a piece of wood that fits inside as a support and cut with a TCT chop saw or table saw. Hacksaw if its sharp but again a support block really helps.