The 'reported fault' is that the window locking no longer works.
It looks to me as though there is a gearbox into which the handle/espag mates. This has a spigot which engages with a latch on the frame of the window. There are also long vertical rods which I presume screw into the gearbox and which more forward or retract, as the window handle is turned, into metal 'retaining plates in the frame of the window at top and bottom.
I think that one or both of these rods has become seized within the metal corner bracket, so that the (cheap) gearbox action can't apply enough force to move it in or out.
I've tried some Plusgas, a brass wire brush, and some mild brute force with a small hammer to try and de-seize things. So far no joy. Any tips from the pros as to what to do next? The window is on the first floor and access is not easy ;-(
Thanks Jon N