Instructions:The e-mail I got says:
Screwfix is offering you an exclusive New Customer discount of 10% off anything you purchase at Screwfix.com, including already discounted products.
Hurry, this offer ends 20th December
To take advantage of this offer just enter the code below in the special instructions box once you reach the checkout.
NOTE: You enter the code as it says on the last page in the special instructions box but it looks like it doesn't apply the discount there and then, but possibly just when they charge you so not sure exactly how it works!
Online version of e-mail can be seen here: http://content.screwfix.com/datalistdec09/E09W45DL3p_B.html?cm_mmc=Campaign-_-E09W45-_-DL3p-_-bversion&source=aw