Sainsburys are selling 6 packs of Brita Maxtra water filter cartridges for half price at £16 but right next to them they had 4 packs for £6. I cannot find 4 packs listed online, so these might be old stock. I bought a 4 pack from Morrisons yesterday and they were advertised at half price at £10 and the bar code is exactly the same but the pack colour is slightly different (paler blue). The printed shelf label said £6 but handwritten beside the price was "DD". Please be kind, this is my first posting.