Use the voucher code FTSL15-1 (courtesy of quidco themselves). It's released on the 9th of July (the day it should be delivered to you).
... don't forget price will overall be even less if you go via quidco.com and the 8% cashback is paid.
I have been looking forward to this game for ages but was determined only to pre-order if I could find a good value deal.
As you're pre-ordering you also get some exclusive in-game content.
Give the free demo a go on Xbox Live Marketplace if you're not sure. Very few things relieve end of day stress like driving through a horde of zombies at 100 mph :)
Vote and comment whatever you wish, I'm sure someone out there will get this bargain and have hours of fun and then my job will be done :)