My friend @whatleydude asked me whether I'd seen any ads with a big X on the London Underground. At the time I hadn't but typcially, the very next day (yesterday), I saw the one below at Old Street.
If anyone knows what it's for or wants to hazard a guess please do. Maybe it'll be followed up by a giant Y and then a giant Z. have you seen any other variations? Is it just limited to Shoreditch or the Northern Line?