The exact same thing happened to my partner at the tesco ATM at the end of Stansted road (Forest Hill/Sydenham border). My partner managed to hold onto his card and tried to run after them to no avail. It was 11am on a Monday morning and they just came out of nowhere. Tried the same technique of trying to distract a Tesco employee that was having his break close by while the other one creeps behind you to grab your card. So definitely be vigilant.