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The type in my photo (cockspur, fnar) are really easy, if you can access the screws. The bit that protrudes over the wedge comes with different thickness pads to screw in underneath and the wedges are readily available in different thicknesses. The one of the mounting holes top and bottom are slots too to help line up with existing holes.
If your handle operates an internal mechanism, then you'd have to be more careful about compatibility.
Like I said, all mine are either fucked, unlocked or completely missing, so I didn't have the removal problem. You may be able to brute force it open. These type are laughably insecure. A Jimmy on the outside and all you're fighting against is screws in PVC. Luckily these are top floor loft conversion dorma and not (easily) accessible.
In other incremental progress. The two replacement velux windows got fitted today, success.
Also, replacement window handles for the smoke stained room of death. How does someone fuck every single window handle in the house?
Edit: will be replacing the yellowed wedges too.