When it first emerged, virtual currency Bitcoin was viewed by many as a fad that would never catch on (ahem, Lily Allen). However, with the value of a Bitcoin rising from around 15p in 2011 to over £600 now, this is one fad we can't afford to ignore. Here's our failsafe guide to what they are, how they work, how you spend them and which celebrities are embracing them (as well as the ones who definitely aren't).