Mark I hadn't seen this thread and as you have bought Nova's material, my comments are now a little late but here goes. I too bought the Nova material for quite a large bedroom window( my darkroom is a converted bedroom) and have had it for about 4 years. My findings are that it is very durable, hasn't torn and is likely to last a lifetime, I think. So what's the problem? Well the material for a large window is very heavy and velcro very quickly loses its stickiness, even though my material is additionally supported at the top by a form of curtain rail. I find that I have to re-fix the velcro each time I use the darkroom by running my hands over the velcro to re-affix. The problem is compounded by the fact that my window is south facing and the material acts like a heat trap. I can actually put my hand near the material and feel the heat emanating like a radiator which seem to further weaken the velcro's hold. I then tried white polystyrene painted black and sandwiched between the window and the blackout material to cut down the heat penetrating the window but with limited success.
If I had my time over again, I'd invest in proper blinds with side channels which can be raised and lowered at will and which don't rely on velcro. Ideal for reconverting the room for other temporary uses as well as the benefit of daylight for toning and even film processing, once in the tank.
If you aren't south facing and if the windows are small, it may not be a problem but I note that you have the force of gravity to contend with. Working only in the hours of darkness may also be a solution especially if there are no street lights to intrude but that's too limiting for a large part of the year.
If you solve your problems great but if you continue to have difficulties and assuming the investment in blackout material hasn't been so high that you are stuck with it then if you expect to be a darkroom worker for some years to come, investment in professional blinds may be worthwhile. Firstcall sell a self assemble kit including channels. You give simply them your measurements, get a quote and the do a bit of DIY. Richards of Hull advertise something that sounds similar to Firstcall blinds in B&W mag.
I hope that all your attempts to solve your problems keep you in completely in the dark. Only on an analogue photographic site could this comment be construed as well meaning and not nasty!
Best of luck
pentaxuser