I too have the same problem.
When this was pointed out to KeyMare on our "snagging" visit, it was met with a shrug of the shoulders!
Having thought about the situation, the only solution would be to install a new shower tray, wider than the current one, then the shower screen can be installed across the opening, thereby, not being impeded by the window, and also allowing the window to be opened for ventilation purposes.
This will be a costly exercise, but one which will be necessary. KeyMare should, in my view, offer to install the shower tray free of charge, due to this being a very poor, planning decision.
My guess would be that hell will freeze over before THEY resolve the situation!!!
Regards,
Lorraine & Peter Braid.12-3-07