Steve,
In my experience a monofilament will scratch a plastic conservatory roof- I know because I almost had to replace a couple of sections that I had cleaned on previously unscratched sections, you could see it happening , scratching the sections in full sun.
Anyone, If you dont believe me clean your car bonnet or worse one of your customers with one and see what happens. Thats why Vikan went flocked. Go on try it and prove me wrong.
There is the possibility that you have a different material in your brush, what make is it ?