But it really is nonsense, and not just for the reasons given by Allister Heath. First of all, what was Big Bang? In essence it was four reforms applied to the working practices of the London Stock Exchange: trading was changed from face-to-face to telephone and computer trading; the fixed commission payable to brokers was abolished, allowing competition between brokers; the demarcation between brokers (who traded shares) and jobbers (who were the market makers) was ended; and perhaps most importantly, it allowed foreign ownership of members of the stock exchange.
The combined effect of these reforms was to spark a frenzied round of consolidation which effectively allowed investment banks to take part in - and then dominate the City. It undoubtedly sparked London's rejuvenation as a financial centre (helped in part by New York's self harm), although it was probably less influential that the abolition of capital controls had been. However, did it cause the financial crisis?
Here's the view put by, unsurprisingly, the Guardian:
The Big Bang was partly about modernisation – ensuring that the City used up-to-date technology such as computers. But it also dismantled the barriers between the separate, narrowly focused firms in the City, the stockbrokers, advisers and "jobbers" who created the markets in shares. Afterwards, all these services could exist under one roof and ultimately, some would argue, it led to the catastrophe of the credit crunch, whose effects the UK is still living through.
I find it hard to plot a course from the merging of stockbroking and stockjobbing to the paralysis in the international debt markets that caused the credit crisis. As a further point, for all that dear old Gordon Brown was ridiculed for all his "started in America" blarney, he did actually have a point: the spark for the crisis was poor lending practices in the States. Lady Thatcher undoubtedly had mysterious powers, but this was beyond even her.
Of course, the scale of the damage done to the British economy is linked to Thatcher's reforms of the City - but not perhaps in the way her detractors are claiming. Big Bang (and the various other supply-side and financial reforms) revitalised the City as a global financial centre. It was thus of far greater importance to the British economy than it would otherwise have been, and a recession centred on the financial sector was commensurately more damaging. You could blame Thatcher for this, I suppose. But it would be rather like complaining that this year's flood was worse than 20 years ago, because now you have so much more stuff that could get damaged.