This is a 2.6 ltr jug with easy flip top lid to fill from tap and 4 week monitor to tell you when to replace the cartridge.
This is the cheapest I've seen for a long time.
Spotted in Tamworth Staffordshire Sainsburys at Ventura Park, they had loads left.
Bought one myself for spare!
Apologies if this is just a local thing but thought it was pretty good given the cartridges are normally £3.50 - £4 each alone!