Onoff

No. There's accepted norms like externally the black stuff is 150mm above a path / patio. Even that can be less if things like French drains are used. FFL can be higher or lower than the DPM. 99.9% of the time this is shown on a section drawing with all the materials to be used and agreed on by all parties before work starts.

I can understand your frustrations but it's a bit tricky sometimes to understand without photos. Most of us would be holding tapes up etc and snapping away. For instance when talking about how my two gate posts were at different heights a couple of pics conveyed the issue to anyone looking:

You said "typically". Thinner screed systems are doable but I'm guessing more expensive than ordinary sand and cement. Found at random: https://gbr.sika.com/content/united_kingdom/flooring/en/products-and-systems/floor-screeds/industrial-screeds/rapid-set-screed.html

I would guess 65-70mm for a sand cement screed. Level with, above, below. Depends on the detail. The FFL can be lower than the DPM if it steps down.

Lopped off the L section just now. Dropped on some bricks, lowered by roughly 25mm. Retained to the post for now by the long M16 stud. Welding tomorrow if the weather holds. Should bring the top red brick up near level with the underside of the post which is what I'm aiming for.

Don't beat yourself up, you may well have ordered the right one. I ordered some st/st brick ties from S'fix the other day for my pillars and the profile is different from that in the catalogue. Happily in my case as the "interlock" if butting them up is better for my needs.

I've been using 2 identical 1/2" socket sets from Halfords for over 30 years first at work now mainly home. Never an issue with them and no breakage though they only go up to 27mm. In later years I have swapped out the commonly used sizes in one set with individually bought, 6 point sockets for better grip on old rusted nuts, on the old, rusted things I tend to play with! I'd also highly recommend a full set of Irwin Bolt Grip Nut Removers like this if dealing with rusty stuff: https://www.machinemart.co.uk/p/5-piece-bolt-grip-set/ They come as a 5 piece set often with space in the case for the "expansion set" which does the intermediate sizes. Shop around. One alternative route if going to the States is to buy the Sears Craftsman set that comes in a slightly bigger red box. Sockets are stamped with the same Patent number. The advantage of the Irwin ones is you can use a spanner/socket or a 3/8" drive. Edit: I believe you lost a set of these as well didn't you? ?

"Tanking" is a liquid applied membrane you paint/roller onto a primed surface. Reinforcing tape is used ag junctions between wall and floor and where different materials interface. This to take up movement. I used the Aquaseal 7.5m kit. This lad shows it being used:

Are you considering tanking behind the new tiled area?

I would in case you upset / break the waste pipe underneath.

How about this: Centre punch the nail heads. Then use a spot weld drill to take the heads off. When you remove the plate just hammer the protruding bits of nail in a bit more. https://www.ebay.co.uk/itm/AK4728-Sealey-HSS-Cobalt-Spot-Weld-Drill-6-x-66mm-Drill-Bits-Sets/312240744435?

If ever that gets done I assume you firstly get it fully emptied then collapse the lid in with a digger? Do you attempt to sanitise the area in any way? Lime over it?

Shows the slide hammer action:

My chippy mate gave them to me. Pretty amazing tbh. They'll pull out angular ring nails from 3/4" ply on joists. The knob on the end you can welt the inbuilt slide hammer to drive the pincer teeth round the nail head.

I'd love to live in a caravan whilst my wife stays in our current house......

What about a nail puller? I've two similar to this. Hoping they'll get the nails out of the ply on the dormer roof when I redo that:

It's Dettol in the bucket! (I really struggle with poo and drain smells. I even wear a dust mask with a few drops of neat lavender oil inside when doing stuff like that). When I took the yellow bung out of the soil pipe in the wall I quickly put the plastic bag over the open end. The bung that had been stuck in the smelly pipe went straight in the Dettol.

What about smacking something like a wood grenade into the slab to crack it? Be a pita if it's got mesh in it. You'd only get localised, easily repairable damage then.

Has your laptop not got Snipping Tool built in under Windows Accessories?

I've pushed a balloon into wastes before then inflated them.

Yee-haw!

Do you have a fork lift or tractor with forks? Jam under the screed and lift! You might achieve the same effect with a 6' pry bar and something to pivot on. Otherwise bfo sledge hammer and see if the screed will crack up in manageable pieces. Assisted by that nice orange crow bar. Then shift, peel of the vcl and reclaim all that lovely pir.

Plastic bag and duct tape. You can get both from Lidl/Aldi. If you take the wc out you might want to invest in a test bung. Best practice overall is have another bathroom to use then you can take forever! ?

Pretty sure that was a Witch Best Buy...

The voice of reason. I like that idea. I like it a lot. ? Edit: The only downside is it will mean multiple room clearouts. I could do with a clear room & seperate storage area to shift stuff to / become a temporary "lounge" or whatever. The logistics always remind me of one of these puzzles ? Rough plan is stairs room next, then hall and large downstairs bedroom (all suspended floor). Large downstairs bedroom destined to be a kid's lounge with attached wc/shower room. Before that happens it'll become the temporary main lounge when I do the lounger / diner knock through. There IS a plan!