6-pack of Brita Maxtra water filter cartridges, £14.99 instore @ Robert Dyas.
Works out at £2.50 / cartridge - cheapest I've seen around lately.
(which is typical as I've just bought the 6+2 deal from Argos though it's only 25p per cartridge difference!)
Argos deal = http://www.hotukdeals.com/item/652394/brita-maxtra-cartridges-6-2-free-fo
Be careful about ordering online though as then you're in for £5.25 delivery charge which kinda defeats the point - best to collect from store if there is one nearby.