Spend £50 instore or online and get a 5p off per litre at sainsburys petrol station voucher.
Whats better is that they are pretty competitively priced in the first place so the extra 5p makes it even better! For example i can get Diesel with my voucher for 119.99p at the local sainsburys station. Unleaded is about 111.9 - 113.9 at stores round here so it would be 106.9 - 108.9 per litre.
I don't know how long its on for, but the voucher is valid for 8 days from the date issued.