Replacing friction hinges in UPVC window - With a twist

A friction hinge broke (the little plastic slider) on a top opening flap.
Looks easy enough to replace - Got the parts, just waiting for a mate to
hold the window for me while I replace it.

But, I've now found another broken one, and yet another that is very stiff
and difficult to close. The problem is, they are upstairs windows, and both
are inaccesible via a ladder.

Is there a trick to changing the hinge with access from the inside only? I
don't want to have to remove the entire frame

Al.