Hi Slid ........... if you're having trouble with mould at this time of year then it's most probably poor extraction. In winter it's usually because the surface temperature of the walls is too low .......... resulting in condensation.
You don't normally need a humidity fan, just a regular 100mm one placed directly above the shower head. Very little will get out through an open window. With the fan above the source, vapour is cleared directly. In my own shower room I don't even have a window, just a rooflight. I've never needed to open it , and have no problem with mould.
I use a shower light/fan combination unit with timed overrun.