Last time I bought some it was from Tesco too. Not really convinced however.
I washed my Aerostich suit in it, it cleaned the suit, and the water did 'bead' well afterwards, but I noticed no improvement in waterproofing.
Think I paid £12 three years ago, but those bottles give loadsa washes for all you kit, and any other stuff you can think of.. I.e. stinky trainers etc.!