@coldclimate I've been reading plenty on this topic for a good few years now but non of my notes are quite organised.

Firstly, I'd recommend this book: https://www.amazon.co.uk/dp/0993077234 even if you know most of it, I feel like it cements things, but if you don't it gives you the words and generally how the different measures work together.

There are many retrofit co-op which have already been referenced, but I'm still not aware of any which operate in the north east.

You mention #PassiveHouse, their lesser standard #EnerPHit is a good term for searching, then there's #AECB which have more attainable standards while still taking a holistic approach.

This whole podcast is a treasure trove but this episode in particular: https://www.houseplanninghelp.com/hph383-a-beginners-guide-to-creating-a-comfortable-home/

Understand the current building fabric, the materials used, and the effects of using non-"breathable" materials on buildings which are. I'm generally for woodfibre or similar first, and then PIR (or similar) when others aren't so suitable. Trades seem to opt for PIR for everything.

I like the guides on this site: https://www.ecologicalbuildingsystems.com/post/best-practice-approach-insulating-suspended-timber-floors (it will look strangely familiar to the wall I'm working on)

Insulation is one big part, it has to be continuous, but air tightness (not just "draught proofing") as such an impact on both comfort and heat retention. Ventilation is another, can't seal up without letting that moisture escape, for that I'm looking into #MVHR, also helps keep down CO2 levels.

And finally, have a plan, with deep(er)-retrofit, everything relates to something else so the sequence is critical.