Around £40 elsewhere.
The link is for a 6 pack of Brita maxtra cartridges @ £16.99
However you get free delivery on orders of £30 or more, so you can get a 12 pack for £33.98 delivered! This works out to be ~£2.83 per cartridge, which is the cheapest I can currently find! There is also 8% quidco, making it a potential ~£31.27.