Bellcast Bead upvc

Not knowingly. what is it? ah, support for render above DPC or opening.

Struggling to get my head around the curves and ends. All the underside of the render needs to be supported so I think it's just a square cut. With a mitred horizontal section what would you do with the vertical bit of bead that is now perpendicular to the wall?

A suitably curved bit of thin board that you can run along the front edge of the Bellcast Bead and a temporary batten the thickness of the render fixed above the bead? The former might need a lip to hook under the bead to keep it running in the same place.

Form the shape, leave to mostly set, remove batten, complete rendering above. Or render down to a batten leave to set, remove batten, render in the bellcast. Or a long former (2' ish?) that can run on the fresh flat render without altering that face but form the bellcast against the edge of the bead, this would be best I think as the render and bellcast are of one piece.

No.

I'd have thought just cut the plastic/metalwork off straight, beyond the sides of the opening, then do the diagonal cutting-in of the render itself, as you can nearly make out in this photo ...

Actually, you're right, the edges are no big deal. Looks like you (or one) could do those with a trowel edge