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Just wanted to get back and say thanks for this tip,
Bought the Soudal sealant and went at it this weekend by opening up the gap a little and getting it all down in there, worked really well - window frame is now fixed in place and the outside noise has been reduced massively!
Actually surprised how easy it was... thanks!
who22
mespilus
You can't 'lock' a uPVC window frame into masonry.
The uPVC will react much more quickly to temperature changes, the frame will expand and contract, as you can see in your picture by 2-3-4mm.
You need to apply an elastomeric sealant, one that can accomodate the movement you can see.
The best elastomeric sealants claim '25% movement accomodation factor', in total.
This means you need for a durable waterproof seal, to apply a bead 16mm wide, and around 8-10mm deep between the masonry and the uPVC frame.
Soudal Fix All High Tack Adhesive & Sealant, (Toolstation 84704) comes in white,
so you save the painting you envisaged.