Not sure if this is all tescos or not, but this is the Glasgow Silverburn location. The compost is reduced from £3.00 to .75, which is a good deal in itself. However, when you checkout with your card, it rings out with the multibuy savings 2 for £4.00, so it takes £4.00 off your total. We didnt notice until we left, but we bought 3 dog foods and 8 composts and I thought it was low when our total was £3.28. I realised in the car, they'd actually refunded our card £3.28 and our total was negative!!!!
Good luck with it and hopefully they dont catch on for a while.....PS, head for a thick cashier ;)