I'm getting a proper draft from this window not forming a proper seal. Is there some fairly easy DIY I can try, or do I need to get a professional in?
I don't need to open this window during the winter, so if there was some artificial seal I could fit that might work.
Given the type of window, I suspect you need to replace the worn out seal with something that has a profile like this one and sold specially for UPVC doors and windows. It should push right into the grooves around the window itself once you remove the existing (and presumably worn out) seal. If my guess is correct, it should seal more effectively than the adhesive brush seals suggested in another answer.
The seals I've seen that have needed replacing haven't perished but they've been permanently squashed out of shape and sometimes have holes worn or cut into them where the frame has rubbed (or feet if it's at the bottom of a door).
You could get a new handle for the top one, to match the bottom one… then you could shim the latch plates.
You might be able to shim the top handle, but it looks like it latches 'inside' the frame. The bottom one goes outside, so will be a lot easier to deal with.
Empty soft drinks cans make great shims. Cut two or three rectangular sections. Use a small file [even a nail file will do] to smooth out the edges so they're not sharp & possibly slim one end to give the latch an easier ride as it first meets the shim. Alternatively, lap the shims round the corner, so the latch rides over more easily. Get some good contact adhesive [Evo-Stik etc] & follow the instructions on the tube.
As they famously say on Blue Peter… here's one I prepared earlier.
This is from a fridge door light, but the principal is the same.
The handles will now close tighter.
BTW, I'm assuming, as those look like generic 'Euro handles', I wouldn't expect to find 'brush strip'. There should be rubber/silicone seal. It's replaceable, but it shouldn't perish or weaken in at least 20 years, so you have to tighten against it more, rather than replace it.
As mentioned elsewhere, you need to be sure that condensation isn't between the panes, otherwise its time for a new unit, as the gas seal has gone. This won't in itself contribute to a draught, but it will lessen the overall heat retention of the pane. btw, you don't need to replace the entire unit to fix the glass itself - those panes snap out & back in 10 minutes.
You could check the condition of the brush weather stripping and consider replacing it.
You haven't explicitly stated where the draft is penetrating so this is just my best guess.
Is this condensation in between the panes of glass? The argon seal might be broken. There is no DIY remedy for this.
