These are definately scatches.
I've been to the customer's in question (Well 3 of them to be precise

). They are all on fairly modern upvc and are kept very clean so definately scratches.
Thanks for the advice guys. I'll nip out to Halford's tomorrow and see what i can find.
As for eliminating this problem completely. Are there any wfp brushes that have a softer head which doesn't leave scratches or is there something i can attatch to my existing brush head to stop it scratching the sills. I know i've scratched a few more but the customer's either haven't noticed or aren't bothered.
Regards,
Lance