On 02/05/2020 15:34, Tricky Dicky wrote:
A photo would help. I would have thought that you would need to create a framework around the window opening and fix to that

Richard

Not clear to me either. For Critall type steel framed windows maybe they
don't need a full studding frame for structural support, just two
vertical or horizontal studs. Perhaps the asbestos was providing some
structural support that isn't there with lighter steel cladding.