Gone West

You can get a 700mm wide electric micro excavator which can be driven through a house.

Yes it was the PHPP 2007 version I used for the initial calculations, £135 from the AECB IIRC. I later updated it to the 2010 version which had data for compact units, such as the Genvex Combi, that I installed. All old hat now, I guess.

Fifteen years ago I did my own PHPP calculations and they were accepted by DDC BC instead of 'as designed' SAP calculations.

There are lots of ideas on https://gabionsupply.com/ Pumice stone, which can be bought in bulk, can be used for a lightweight filler.

There are examples of thin gabion walls on the Fine Mesh Metals site. They are called zenturo panels. https://www.gabionbaskets.co.uk/gabion/gabion-fencing-system

This calculator reckons 12 tonnes, unless I've done something wrong. https://www.primaryaggregates.co.uk/aggregates/aggregates-calculator

That's right. We had an EPC A and when we sold our place we had people bidding against each other for it and we got significantly above the asking price. We were told it was because there weren't PH on the market.

I think it is fairly straightforward, if in doubt, don't do it.

You can have Lindab with a paint finish on top of the galvanising. We had Lindab with western red cedar cladding and fascias with no problems.

The third house I owned was dot and dab and taped and it looked fine.

I had a similar situation and used concrete posts and concrete patterned gravel boards, but it was only 10m long. I have another area about the same length where I am using UBs and oak sleepers.

I have lived in a large number of different houses and I agree that a traditional wet plaster finish feels best. If you can find a good plasterer who can do a good job with a bonding coat and a finishing coat then go for it. Finding a decent old school plasterer who can do it though might be difficult.

For a soakaway, have you come across the idea of crates with 'bio filter media' in them to further reduce the toxicity of the effluent. IIRC it was something that I discussed with my supplier many years ago.

I don't know what the actual requirements are, but when we installed our STP in 2010 we discharged to a Klargester reed bed system. Several years later the supplier said the effluent could be discharged into a soakaway. As the reed bed system was quite high maintenance we installed a soakaway.

I wouldn't use a Biodisc. The local farmer had one and it was always breaking down, and it's a grim task sorting it out. There are plenty of STPs with air pumps, which is what we used. Good luck with the job and I hope it finally gets sorted out for you.