I think (but not entirely sure) that you’ll have to spend more than £6 to fix it, as it’s too much arsing around to fix the screen alone and you have to buy the digitizer thing with it – so you might be looking at £30-40 ish I think.
I used this crowd: http://stores.ebay.co.uk/fixmiphone when we both broke our rear screens on our 4s. Their service was good and the screens appeared to be original parts. I notice they don’t have any 3GS screens for sale, but it’s worth dropping them a line and asking as they may have some in stock.
I’m no expert in screen reapir, but mrs deadly has just smashed her front screen, so the above is what I picked up while trying to find a replacement part.