Phone repairs?

Someone on freecycle has very kindly given me an iPhone 4S, in good working order except that the glass is badly cracked in the top RH corner. The actual LCD seems fine, everything works, etc., it's just the glass so far as I can tell. Having looked at the price of the kit (tiny) I was tempted to try DIY, but the web sites that show how to do it are scary - I've decided to chicken out and just pay for someone to repair it.

Can anyone point me at a phone repairer in central-ish London that can do this without messing it up and won't charge an arm and a leg?