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I've drilled through the edge of window frames (holes parallel to and in between the double glazed panes) in the past. One hole at the top, one at the bottom, ventilate with an aquarium pump for a few days, re-seal with self tapping screws and rubber washers, remove the screws to dry it out again most summers...
MrE-
Good point, thanks for that - I’ll try the frame first, holes will be discrete and less chance of a critical fail. Looking at the frames again, I think it’s steel - exposed metal hasn’t gone that dusty dull shade that alu does after a few decades.
If it's toughened glass, don't try drilling it. It will shatter
Definitely want to avoid this! I’m 90% sure it’s not toughened, there’s no markings and I’d guess put in in 80’s/90’s on lowest budget. I’ve noticed one has cracked in another flat, that’s gone in the way a greenhouse window cracks.
Thanks for the advice all, think I have a plan now; drill the sides of the set-in panes, and replace the opening windows with new.
Chopsicle
Not as far as I can see - they're box-section alu frames, the glass slid into the 'H' section, with five + windows built/installed in one frame. I think they might be done like this because the building gets an absolute battering from the wind (hence the blown seals) and this prevents any chance of them working free.
....but, I could replace the opening windows (2 out of 5 panels) - and put up with the rest until they all get replaced in a few years.
The alu frames are ridiculous for thermal bridging/heat-exchange, so the need for proper vacuum sealed double-glazing is negligible.