I rang the apple store in regent street and they said if they didn't have the part they'd have to order it in which is a bit of an issue as I'd have to travel down to London for a day in the hope that they had it in :/
There is a very slight dent on the back of the screen ( as in the aluminium) but it's been there for months, and doesn't correlate with a crack on the actual screen.
You'd probably have to leave it with them anyway - they wouldn't be able to repair it on the spot, so you'd have to leave it with them, wait for the part/repair, and pick it up.
If that's a hassle, give AppleCare a ring, and they can pick it up for service. If you want quicker service, avoid the popular/busy stores like Regent Street/Covent Garden.