Bert Coules wrote:
The Natural Philosopher wrote:

Have you got, or can you borrow, a router?

That's a brilliant idea. I wouldn't even need that big a cutter: I
could make several side-by-side grooves, slightly overlapping, in the
inner face of the frame and widen the opening that way. And the window
isn't as deep as the timber (timber 100mm, window 60mm) so I needn't cut
away the entire depth. In fact, it would give me a lovely secure ledge
to bed the window onto.

I wonder if my cheapo router is up to the task? I'll find out tomorrow.

Many thanks!

60mm should be OK

use it like a planer and go full depth and a few mm at a time gradually
moving inwards, If you **** up,. go further and take less off the other
side.