I know this isn't a deal that you'll go rushing to the shops for, but was more just a surprise when I saw it on the shelf- It looks like a mis-price, but sure enough sold for £2.50.
On the shelf was the 250ml bottle, priced at £2.50, the 500ml bottle, for £2.50(!), and the 750ml bottle for 7.something. Given that bigger bottles are generally better value, the fact that you can buy two small bottles and get 1L for £5, over 2 quid cheaper than the 750ml, is, pretty good.
These were normal price tags, no apparent promotion- So possibly an error. Still, they scanned at this price, so all is good :)
This was in the Asda superstore, Luton. I have no idea if it's National.