I have contacted them and they replied with this:
Dear [NAME],
Thank you for shopping at Overclockers UK -
www.overclockers.co.uk
This e-mail includes full details of your order and other important information.
----------------------------------------------------------
Order Number: ########(Internet Reference: )
Date ordered: 06/02/2014 09:54:36
Payment Method: Zero Value
Shipping Method: Royal Mail 1st Class
----------------------------------------------------------
Goods Ordered (prices in GBP)
£0.00 x 1 - AMD Battlefield 4 Game Voucher - ONLY for AMD R9 Series!
Sub-Total: 0
Shipping: 0
VAT: 0
Total: 0
I thought that they would just email me a code? I may have missed the voucher so what does it look like as I may have mistaken it for something else.