“I had an environmentalist in the back of my cab once,” is not a line you often hear from a London taxi driver.

Black cabs used to be among the smelliest and noisiest vehicles on the road and the pump-and-brake driving style favoured by many cabbies is hardly designed to minimise emissions. But the cab-maker Manganese Bronze has been trying to clean up their act and next year will start to offer a plug-in electric version. This should be cheaper to run, but the vehicle will cost more than the diesel model - between £30,000 and £43,000 – or the equivalent of several fares to Heathrow. For other road users, the downside may be that the new cabs will be as quiet as a milk float. “Oh dear. Didn’t you hear me coming guv’nor?”

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