All Hijacks welcome:)

@djh: who does the path on the left belong to? Neighbours. Let’s say no more. But on a related issue, it’s never been at all clear to me, how (in typical UK law regarding ownership deeds which of course say absolutely nothing on this subject and rarely would), who owns the land beneath the eaves, or whether ownership ends on the external skin of one’s own building? - Encroaching ~ 130mm + onto a path , (even beneath one’s own eaves), may become a practical impossibility for many, and a irreconcilable problem, despite our deepening climate crisis. I have no answer, and would welcome learned opinions on that.

“..whether you fit a mechanical ventilation system and what type will affect your windows - whether they need vents or not.” Indeed yes. My previous 3g suppliers for the rear side were surprised at my need for trickle vents. I took the view that I could always seal them in the future, and they have proved their worth in the meantime, and have not negated the further benefits of the 3g.

@Tony “Are you doing new windows? 3G ? I would fit new windows outside the reveals overlapping by 20 to 40mm then wrap the EWI round them covering the edge totally and front of the frame as much as possible.
..Windows do look big so you could use say 50mm pir or xps right through from face of new render to inside edge of reveal then use eps for the EWI” Yes, that had crossed my mind, and I know it’s commonly done. But fitting the windows outside the reveals might be a mistake when the mediocre thickness of EWI will only represent a minor proportion of the insulation. It would work I guess if I were to wrap 50mm right into the reveals and then fit smaller windows. This is a strong possibility and a win-win since they are larger than necessary.

@Jeff B “I have the added problem that there is practically zero overhang of the roof!” Yes I can see from your picture. That complicates it. I guess that it would need an external timber “ladder” bolted below the verge, then fit a dry eaves kit to the slate extension. It would be necessary trim back enough depth of the existing wall (~30mm) to insert battens to the wall top. This would leave behind a kind of “vertical” cold-bridge (the existing wall to the slates), fixing that one gets bigger. (And I agree with comments re Thermodynamic, - I can’t see how they won’t just ice up in UK humid climate. A drier air/more continental location then maybe better.)
“P.S. Apologies to Justin for hijacking this thread but hopefully some of these issues will be of interest to him too.” No Apologies necessary, same here. I can finish dryline paper board joints nearly invisibly now having learnt on my own house doing IWI, (It’s how you twist the trowel!) but large plaster/render areas,,Hmm (Than you JamesIngram for the encouragement)

@Jamesingram: “do you mean the detailing join from EWI to face brick/cavity wall finish ?” Yes I do mean this. I think your suggestion of beading it up is practical, and I could use that on the corner since I’m “not allowed” round the end elevation to the left.
“.. a side note , I've been building my window extension boxes (into EWI) out of 12mm cement board (aquapanel/hardibacker) as I've felt the need for a fire proof barrier internally between the window frame/ reveals and the EPS EWI..” Yes, I like the idea of concrete board. All this foam, especially internally bothers me and fire resistance has been a big topic recently. I have tried as far as possible with my IWI to close gaps, but that’s not possible in places like first floors timber floor levels without almost 100% gutting a building.

“..I can’t plaster or render anything larger than a few m2 if you want it flat but the thin mesh coat renders are very forgiving and in fact far easier than plastering or traditional rendering. if you can use a hawk and trowel (the flick onto trowel from hawk move) you might be ok.”
That is encouraging, and I might even give it a go on that basis! Your work does look smart and has some great pointers there, thanks for the images. I must investigate that

Justin - apologies, have been away for a week or so and just catching up on stuff. Thanks for your comments. I still fancy the ready-made aluminium profile to "cap" the EWI. OK it may not look as nice as an extended roof but I could live with that and with the relatively small area of wall that would be a cold bridge. The cost of gable ladders and subsequent roof alteration would add significantly to the cost. I do need to insulate the inner wall of the gable end though, which is straightforward and not expensive. The tricky bit is insulating the top of the wall! I could maybe get some expanding foam in there. It is a cavity wall (approx 50mm cavity) with blown rockwool fibre in it.

James - thanks for the ideas and the sketch. I do know a friendly local roofer who has done some work for me so maybe I should at least make an effort to get a quote!