Wow, that's the chapter and verse for outboarding windows then - v gd.

While you're at it, I'd be ever so interested to see the frame pushed
a) entirely just outboard of the masonry i.e. corner to corner with the masonry, with that majonry jamb insulation removed (that's how I've been doing it). and
b) as that corner-to-corner is prob a weak point, frame pushed yet further outboard, so say 30mm away from masonry outside face.

These wd of course require thicker EWI - but EWI reveal depth, back to window outside face, need be no more than say 1.3 x the EWI's overlap distance across outside face of frame. So if overlap is say 25, then increasing EWI reveal depth greater than 35 won't improve the numbers. Tho, with these open-in windows, overlap cd easily be 50 or even 70, so reveal depth greater than 35 wd then be worthwhile.

So here's my updated detail. We're going with Ecocontract triple-glazed timber windows, 0.9 u-value (added benefit with these windows is that we can also overlap the insulation at the sill).
1. Is it realistic to bend the straps 90 degrees like that? I want to do that in order to fit the XPS insulation snugly.
2. What screws should I spec to fix into window and into the masonry?

Any ideas on how we might protect the windows once they're installed if we don't install the insulation for a while?

Shevek; Ive just installed a window, very similar in design to yours with insulation surrounding the reveal and the window mounted itself into the insulation. I am intending on doing EWI, but probably not for at least a year yet. The only difference is that my window is flush with the wall and so i was able to use normal screws directly into the blockwork. At present the outer part looks a mess as the insulation surrounding the frame is visible and the sill is huge! But it is watertight (ish) until the new EWI is completed next year.