It’s four years and one week since I got an email from leading bus design company Capoco agreeing to put together a proposal for a replacement Routemaster, a New Bus for London. The feature - which ran in the Christmas 2007 edition of Autocar - was in response to then London Mayoral candidate Boris Johnson’s call for a replacement for the classic double decker.

Many people had mocked Boris for even suggesting the idea, but I went to see Johnson in his campaign headquarters with the Capoco proposal in hand. I said in the feature that public transport needed to raise its game in terms of design and told Boris that building a new cutting-edge, super-green, bus specifically for the capital was entirely possible.

Johnson, possibly rashly, put the commitment in his manifesto for the 2008 election, promising to get a new Routemaster on the road within four years of becoming Mayor. This morning, in Northern Ireland, the first production version of the ‘New Bus for London’ - in concept very similar to Autocar’s proposal - came off the line at Wrightbus, three years and seven months after Johnson became mayor. It’s the first of eight proving vehicles that will be real-world tested in London over the next six months. This example will arrive on the streets of London for driver training in mid-December.

So what’s the big deal? If you appreciate engineering, this is mighty piece of work. It’s built around Wrightbus’s own bolt-together aluminium spaceframe technology and is powered by an electric motor on the rear axle. Power comes from a 4.5-litre diesel generator, which drives both the motor and re-charges the 17kWh battery pack.

Recent tests at the Milbrook test track have shown that the range-extender set-up produces a fuel economy of 11.6mpg (640g/km), compared to the 5.8mpg (1295g/km) for a normal diesel double decker and 8.6mpg for current hybrid buses. Emissions of Oxides of Nitrogen (NOx) are halved.

The diesel engine and generator are cantilevered off the back of the chassis and the curved rear of the bus is made from an advanced structural composite usually used on yachts. The three doors and twin staircases will make it the quickest-boarding double decker ever seen on UK roads.

The other really impressive thing about the bus is the attention to detail inside. The Hetherwick design studios has designed the entire interior from scratch, making it look closer to the inside of a Boeing than a typical bus. From the cork decking on the rear deck to the soft LED downlighting, this is public transport as it should be.