Christensen had a head of dark hair when he helped to expose the Jersey currency scheme, which resulted in UBS's Jersey unit and accounting firm Touche Ross & Co. -- now Deloitte -- paying almost $40 million to settle lawsuits. Regular bike rides keep Christensen as trim as two decades ago, but his hair has turned pearl white.

He said: "I was set. We had a pretty good lifestyle and plenty of friends."

But he was forced to move to London as a result, where he now fights governments and campaigns against financial secrecy, including on his home island of Jersey.

He founded the Tax Justice Network in 2003 for the purposes of pushing greater regulation of tax havens. It's estimated that $5 trillion to $32 trillion is currently stashed offshore for tax purposes - this is about a third of the entire global domestic product. Christensen thinks the number is at the "top end of that range".