Replacing Window Fittings

Hi, I have got PVC sash windows in my flat and I'd like to replace the existing locks with brass ones to make the windows look nicer. However I can't work out how to remove the locks, there are two screws at the top which remove the lock mechanism but the catch seems to be secured by two on top and two on a right angle bracket which I can't get too without taking a window off it seems. Can anyone advise? Also, the brass ring lifter which were left on some but not all of the windows at 1.5mm thick and I can't seem to find any of the same anywhere. They need to be that thin as they slot inbetween the frame and the window when the windows are closed. Does anyone know where you can buy these? I can provide photos if that is helpful.