Yes it's Brimstone Ash from Vastern timber. Expensive but really excellent quality - straight as a dye, virtually no knots, and it came really well packed. It cuts cleanly and we haven't experienced any splitting, despite Vastern warning that it can be brittle.
We originally intended using western red cedar but discovered that cedar reacts with zinc and we have some high cladding above our zinc roof. The Brimstone Ash is inert (so we are told!). It's about the same price as Canadian cedar but has the benefit of going silver quicker, and it's British, so lower transport CO2 costs.
Stainless nails are an absolute must. We're using Timco FirmaHold 16G 50mm Stainless 2nd fix nails through the face - very hard to see.
Regarding the edge above the windows. Although it doesn't really show in the photo all the bottom edges are cut at a 30 degree angle to leave a drip point at the outside edge. This was recommended by Vastern. The same for any joins in long verticals, though incredibly I think we have only needed 2 or 3 due to lots of long lengths being supplied and careful selection by our carpenters. For the bottom edges they have fixed them slightly over sized then run a rail saw.
Anyway we feel the angled cut at the bottom should stop water pooling on those bottom edges.
Further progress on the cladding today: