Harry Bloomfield, Esq. wrote:
Upper floor is smaller than the ground floor, so there is a triangle of
space below the windows, maybe with a 2 or 3 foot base.

He cannot add insulation inside plasterboard walls, because of built in
furniture. What is the usual method of insulating such spaces please?

We've just bought a place like that. There's fibreglass insulation between
the ceiling joists of the ground floor, and the vertical walls of the first
floor are backed with fibreglass batts too. I'm not sure what the state of
the upper roof is like, but we'll find out next time we have to replace
the flat roof on the dormer. There are hatches to access the spaces.

I'm pondering putting some insulation between the rafter (with a suitable
air gap from the tiles) to add a second layer (partly to keep heat out), but
will probably want to see how it works over the seasons first.

Theo