In July 1983 an agreement was reached between the British government and the London Stock Exchange that was to transform the London securities market in October 1986. The result was a revolution that extended all the way from the design of the market to those permitted access to the governance of those who traded. Given the speed of change, it was termed ‘Big Bang’ by contemporaries. As the scale of the transformation became known, politicians were quick to claim credit for what had been achieved. Subsequently, Big Bang has been regarded as one of the crowning achievements of the Conservative government led by Mrs Thatcher, providing the foundation for the City of London's re-emergence as the leading international financial centre. However, only now is evidence emerging of the intentions of those involved at the time rather than as expressed in the light of hindsight, which casts doubt on what they were seeking to achieve, whether viewed from the perspective of either the politicians or those in charge of the Stock Exchange. Instead, the longer-term pressure for change in the London securities market, the chain reaction caused by abolition of fixed-commission charges, and the proactive role of the Bank of England appear to be of greater significance. What an examination of the process leading up to Big Bang in 1986 reveals is the complexity of political decision involving the financial sector and the danger of unforeseen consequences. One small change can create a tipping point if the result is to remove an essential element within a complex system.