well it's just a guess... on the two outside walls I mean (there are other walls that adjoin other flats, or the inner courtyard). There are two main room, living room/kitchen combined, and bedroom. the bedroom is all glass. The living room has glass for most of the south facing wall, maybe a small section isn't. And then the west facing wall must be at least 50% glass. The rest of the flat is bathroom and corridor. The walls that are glass are pretty much floor to ceiling, either doors, or full length glass, or obscured glass, or I guess there is a section that isn't actually glass but has a sort of opaque finish but is the window. So probably just as much heat through it.
The internal walls are plasterboard, but there aren't many of those - but the main one happens to be between the main windows/doors to the west, and the front door, so preventing cross ventilation.
I don't know why they didn't do more when the flats were being built. I assume that the other flats aren't as badly affected - mine is smaller than most, has the layout that prevents cross ventilation, has two faces that are glass because of corner situation, has south and west aspects, and is ground floor so windows can't be opened at night. I knew is was hot when I bought it, but not this bad - and it's not like I really had any choice anyway, was a desperation move.
I wish they'd do more now, and I'm trying to complain that the flat isn't built to standards fit for purpose, but I doubt I'll get anywhere.
Overheating is a real health hazard, though, and I've read lots of evidence to support that. But it's hard to claim that actual building regulations were breached.