Thank you all - real food for thought!
They don't like blinds because they sleep with the window cracked open for most of the year, and say that from experience, a blind gets blown about and rattles. They also say that light seeps round the edge of a blind.
Swing arms are new to me (well, I must have seen them on the odd occasion, but taken no notice!) and I think that might be the answer. I need to check measurements.
A car breakdown prevented me from seeing the house myself, and the photos are not good. I am glad that I checked in with you as I can now go to look with these ideas in mind.
It is not a conventional dormer window, although that might have been the best way to describe it from the inside. One good thing is that the spare room has the same set up, so I can experiment with cheaper fabric (possibly even some of my remnants).
Thank you all, and I will let you know what gets decided, although distance and family commitments mean it won't be soon!