I am sure I saw this a while ago, but can't find it... I have some glass which is scratched, is there a way to polish out a scratch in glass?

Jewellers rouge is the stuff you need, but if you can feel the scratches with your fingernail, you will not budge them

halfords do a typically overpriced windscreen repair kit.... but I don't know how good it is.

Frost do a product to polish out a scratch. Below is the link.
Regards,
http://www.frost.co.uk/productList.asp?catID=20

As Mark Allanson said, if it's a fingernail snagging one forget it. The Halfords kit is to address stone cracks rather than scratches. I have loads of diamond paper (1-5 micron IIRC), used for polishing optical fibres, which will do the job but you have to be patient it takes a while (ace for watch lenses, even crysta). Anybody want a few sheets send me an SAE, I'll send a selection from what I have.