Before anyone asks...
Big house, 7 bedrooms, located in Kenton (North West London). Exterior
paint-work neglected for many years, now affecting rendered walls and
most woodwork both. Interior not much better, having last been
decorated some 25 years ago, albeit to a high standard when done.

I have my work cut out, as you can imagine.

One of my tasks was to repair the exterior cill to a metal-framed,
stairwell-landing window, to the extent of chiseling the rotten cill
back to sound wood and then splicing on a ripped-down length of new
cill moulding, recently bought for the purpose.
Unfortunately, it turned out that the whole frame was shot, beyond
repair, and I have had to remove the whole lot and board-up the bare-
brick opening, temporarily, while I source a new frame.

My next problem (and the reason for this post) is that, the heads of
the screws fixing the metal frame to the hard-wood frame, are obscured
by the glazed, leaded panes of glass, which were only recently
renovated! The upshot is that I can't access these screws without
removing the glass panes. This I do not want to do!

Removing the window wasn't a problem, as I just mullerred the rotten
wood around the metal until it all came away. Any solid wood still
attached just got removed by drilling, cutting and chiseling around
the remaining screw threads, still in place.

However, as you can probably figure, I now need an alternative way of
re-installing the metal frame back into a new wooden frame that I will
have made. As I will be unable to fix "through" the metal frame, into
the new wooden one, I have been thinking about having a specially-
made, hardwood frame, that could "sandwich" the metal frame between
'inner' and 'outer' components, that are independently screw-fixed
into the brickwork. Granted that the situation is further complicated
by the metal frame being rebated on the internal side, with a flange/
collar/whatever, visible from the outside... to prevent drilling
access the original through-frame screw-fixings I presume!?

I fairly sure this could work, but before I commission the joiners,
I'd like to know if anyone else has experienced the same, and if there
is a more common-practice method that is widely used for solving this
issue. I don't want to over-engineer this and incur unnecessary
expense.

Looking forward to your responses.
Many thanks.