Bitcoin: it’s the crypto-currency which has no physical manifestation, but which has spectacularly soared in price over the last year. It’s brought vast wealth to a few individuals, just a thousand people are thought to own 40% of the entire market. It’s got a bad reputation too, described variously as a massive scam, a dangerous bubble about to burst, and a medium for some to illegally purchase guns and drugs. But it has perfectly legitimate users too.