If its just the locking system thats faulty then it should'nt be a
problem to repair.
Maybe local double glazing company would take it on, but if its not a
system they work with they me not be interested. (could be expensive).
To do yourself
Take some notes off the Dimesions of the locking system and handle
(backset size normally 35mm flush side off plate to C/L off spindle),
ask some local pvc fabricators/double glazing company if they have the
parts (complete locking sytem and keeps).or any hardware suppliers
that could sell you the parts.
The again you could ask one off the factory fabricators to do it on
the side, Prob be cheaper and knows what to do