I have 3 codes for Screwfix but this morning found out that they stack to make a better deal.
Code 1: Spend £20 get 10% off - SFL127A3V1
Code 2: Spend £25 get 15% off - SFL127A3V2
Code 3: Spend £50 get £10 off - SFL127A3V3
I added an item to my basket to get to over £20 and applied the first code to get 10% off. I then added another item to get the total (with previous discount) to get over £25 and applied the second discount. Not tried the third on that though. This gave me a bill of £30 on a £40 spend. Codes are valid until 27/11/2016
Edit: This only works for specific accounts. You must have been sent through the promo codes in the post.