thanks all for the replies. I automatically told her the brush was scratch proof and as it was only 1 window and no others i couldn't see it being possible.
They have been good customers since i started so i did go back today and have a look and i told her i'd hazard a guess and let her know.
There were 2 very fine scratches pointing different ways, but i was none the wiser because her husband had a go after i'd gone at cleaning it again to see if it would disappear, so there was no evidence for me to come to any conclusion of what else might have been the reason. But i told her i would enquire with others before i got back to her.
I've got to add that she didn't accuse me, just mentioned it and asked me if i had ever scratched a window before with my kit.
I looked at robs link and others to see if there was a cheaper old wives tale of ways to get it out. I found this link about clear nail polish, which i will mention when i call here tomorrow.
http://www.hometips.com/home_probsolver/hps08/05_windowsandscreens/hps_windowsandscreens02.htmlThanks again.