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I'm going to take an armchair punt here too!
I would measure the depth of the beam under the window frame, use a track saw with depth set to 19mm and multitool to finish the cut at each end. Pull the ply off the beam and see if the window opens.
If it does open, put a stud between the rafters under the beam and screw the ply to it, tidy up the edge of the ply with some small beading to stop condensation collecting there and decorate.
If it doesn't open then remove the whole piece of plasterboard and put stud recessed in the rafters and put the ply back on top of the stud.
In answer to your question about the multitool, yes, definitely a useful tool tackling this kind of job.
I would use a track saw to cut as much as possible because it will be a lot neater and quicker but it depends if you have one available and how easy is it to hold it up there.
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Airhead
stevo_com
There's a rebate built into the sides of the frame (arrowed), for the surround to slot into. Accepts 12.5mm plasterboard or ply. If there's any scope for un-bogging that.
As Howard says, are you sure you want to open them now?
The vent flap shouldn't cause too much of a problem, but id be cautious unlocking and expecting the pivot hinges to open and then close again.