This is a bit long-winded so please bear with me... (if you're too lazy to get a bargain then stop reading now :))
Up until yesterday I had been buying Purdey's drink from Tesco who were selling them for 2 for £1. Bought another 2 today but I was charged £2.90 even though the SEL still said 2 for £1. Went to customer service expecting to get £3.80 back for the error but the dumbo was adamant that I was owed only £1.90. Checked on the Price Promise website and found that it compared to Asda who are selling it for £1 i.e. difference of 90p.
So check your stores to see if the old SEL is still there, add another 9 filler items then get 90p price promise for every 2 bottles of Purdey's you purchased. Worst case scenario, you get £1.90 back which means they'll cost 10p for 2 or ideally you get £3.80 back i.e. double the difference and in profit! :)