One of the window handles (espagnolette) on a conservatory fanlight
would not turn. It wasn't locked, but pushing on the central lock
button, which usually releases the catch, didn't work. It would turn
maybe a millimetre, but that was it. I tried it with the key in to see
if that helped, but no joy.

I had a couple of spare handles, so examined them to see if it was
possible to access the catch. I tried pushing in various thin pieces of
metal and needles to see if I could get to the plastic piece which
wasn't releasing, but that also did not prove possible.The design of
these handles doesn't help; to aid security, only one screw is visible
and removable when the handle is in the closed position. The other is
hidden under the lock, so it is not possible to remove the lock from the
window unless it's in the open position - Catch-22!

In the end I was able to get a junior hacksaw parallel to the handle to
cut off the small "lip" which stops the handle moving too far when
closed. That just allowed me to get to the plastic piece, push it, and
turn the handle. This is a close up of the handle after I removed it
from the window showing where the plastic had snapped:
https://ibb.co/qmdbkd1

--

Jeff