In October I blogged that you DO need to show your boarding pass at duty free. Over the summer I was involved in a campaign to tell people not to show their boarding passes in airport shops, as there's no requirement to do so – and shops save money if you do, that they don't (but should) pass on to us. Yet the rules are different in World Duty Free shops.

You don't need to show your boarding pass at airport shops, but you do at duty free. A media furore over the summer has sown confusion, so it's time to clear it up…