Found these at my Tesco garage in Ipswich (Copdock) and snapped 5 of them. They smell quite nice and last for 2-3 weeks. Oddly, right next to it there was some others (the blue foot) at their normal price of £2. They were also priced at £2 in store at Tesco so it could be a special price for garages. Full price at Tesco.com as well. Might be worth taking a look next time you refill!
Ocean Surf Dolphin Air Freshener for Cars, 18p at tesco garages (normally £2)