Find a suitable spanner that slots over the square part (or use an adjustable spanner as I did) and then try winding the window using the spanner.
If the window is just a little stiff/ slight rust within the winding mechanism, the extra leverage helps to get it turning again and once going things start freeing up.
(also having the mechanism in rust remover for almost a day to try and release the stuck winder appears to do absolutely nothing).
