Previous owner fitted a mock sash with fixed lower window and upper sash top hung on butt hinges. The upper sash is out of square with the frame with a sizeable gap on one side and tight on the other. Any thoughts if/how this can be straightened out?
Not much you can do to straighten an out of square window frame or sash TBH. It would (presumably) have been made square, so possibly worth seeing if you can pack off the left hand hinge a bit using a 1mm (green) plastic glazing packer, or even a shim cut from a Cornflake packet to the size of the hinge leaf (always assuming the sash was hung on conventional butt hinges). If that isn't enough you might need two packers or two shims of Cornflake packet. Amazingly useful things, Cornflakes packets
The diagonal are approx 28mm different, so it’s definitely the frame. I might see if I can shim between the frame and the wall as well as the LH hinge.
In order to make that work you'd need to loosen the screws holding the frame to the masonry, plus cut the sealants on both the inside and the outside - and re tighten and reseal afterwards, of course. Also bear in mind that the frame has a solid head conecting the two stiles which is going to resist any attempts to adjust the frame. Shimming the sash hinge is probably a bit easier, to start with, but I think any work you do will need to be on that sash
Can't see that frame moving very easily to be honest. Always an easy way to tell if a frame is out of square that has a double glazed unit in it is to see if the silver spacer bar follows the frame, from the small area I can see from the picture the sash looks square so it's likely to be the frame, how has it gotten that far out, 28mm across the diagonal is rather a lot, surprised the sash even shut
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