Please help! Window frame/wall gaps

Hi,

First time poster and complete DIY novice here. I was wondering if anyone has any advice for a problem I have with my double glazing. Basically the top right corner of my frame has come away from the wall leaving about a 1cm gap. U can actually poke a small screwdriver through the hole and outside!

The gap runs along the top and down the side in an inverted L shape and the frame can be physically pushed outward with very little force. I've done a little research and the most popular suggestion for this type of problem appears to be expanding foam, but can anyone tell me would this be suitable for a hole of this size? And would it secure the window sufficiently to stop it moving when pressure is applied?

Thanks

Yes, be careful how much you use though, it really does what it says on the tin.