Haha, that's fair enough - thanks for the explanation :-)
The curtains are a piece of stock footage (and I would imagine have been created using cloth/dynamics simulation in 3D software), so are just a 'flat' layer (and aren't perfect for that reason) - I considered creating them myself, but I was short of time (as always!) and they were such a small part of the video that I decided to compromise.
Cheers,
Phil