LONDON VAUXHALL DEALERS

Another woman shouted at the mayor, urging him to resign.

Mr Johnson struggled to make himself heard as he said: ''Tonight we are going to have a huge number of police on the streets.''

He added: ''It is time that people who are engaged in looting and violence stopped hearing economic and social justification for what happened.''

The mayor, who was eventually guided away from the crowds and television cameras, followed other leaders by ending his stay abroad to join efforts to quell the violence that has blighted London.

The move came despite Mr Johnson's aides previously insisting he could deal with the burgeoning crisis remotely as if ''he was sitting in his office''.

Mr Johnson arrived at a cordon outside Clapham Junction station where members of the public gathered.

He was to greeted by the sight of a burnt-out fancy dress shop being hosed down by firefighters, surrounded by litter strewn on the streets and smashed windows.

Crowds of people armed with brooms waited for permission to enter the cordon, which covered St John's Hill and Lavender Hill, to start a clean-up operation - a movement started on Twitter.

This doesn’t mean that the Ampera will not make it to the top like those Edwardian cars on the Cairnwell Pass, but the Vauxhall’s engine can only summon a maximum 85bhp and dragging the 1.7-ton Ampera uphill without battery assistance is going to be a long, slow ascent.

To prevent this happening, the Mountain Mode program needs to be activated 15 minutes before the ascent. This keeps the engine running for longer than normal to give sufficient charge to allow full performance all the way up. Subsequent models might include some sort of predictive navigation software that could activate Mountain Mode in good time if it determined a steep ascent ahead.

And steep they have to be. GM’s European engineers spent the past year driving up and down Alpine passes without finding a single one that requires Mountain Mode. In the US, engineers only required it on a couple of ascents up the Rockies.

I had high hopes of the Cairnwell Pass, however, particularly if we drove spiritedly to use up the battery power.

Harry Inglis’s Contour Road Book of Scotland describes the road from Bridge of Cally as “rising 1:17 followed by several undulations, with quick turns, mostly 1:13”. From the Glenshee Hotel, the route passes through “uninhabited country, and after three miles, clambers up the hillside”. The notorious 1:3 Devil’s Elbow, near the end of the 18.7-mile ascent, is bypassed these days on the way to the 2,199ft summit. Frankly, the Ampera breezed it.

There was more than enough power from petrol and battery reserve to accelerate strongly, even at the end, where we needed full power to overtake a Polish artic. We covered the 18.7 miles in 29 minutes at an average 38.7mph, although ascending 1,699ft from the Bridge of Cally took its toll on the battery and we ended with just four miles left of EV mode. Then we drove around to use up the last of the EV mode and attacked the slope with equal gusto. To make things tougher, we put on the air-conditioning, headlights and radio. Without the Polish truck to negotiate, we climbed faster and arrived in 24 minutes at an average speed of 49.7mph.

And while the Ampera never ran out of power, the engine was labouring as we approached the summit and flooring the throttle was the only way to access the dregs of battery power to speed the climb. The reserve was there, but there wasn’t a lot left.

So the Ampera conquered the Cairnwell Pass and in doing so showed how over-engineered and charmingly first generation it is. This is the stiffest incline test that an Ampera is ever going to face in the British Isles, so is Mountain Mode necessary? Put it this way: its inclusion is a measure of just how much GM’s engineers want this car to be a no-compromise solution.

Electric driveline explained

The Ampera consists of a 16kWh, lithium-ion battery, a four-cylinder, 1.4-litre, 85bhp petrol engine, a main electric motor, a secondary electric motor/generator and an epicyclic transmission.

Four main modes of operation are mainly to do with the interplay between the two electric motors. Single-motor electric-vehicle (EV) mode is at speeds below 60mph where the main battery supplies its 149bhp/273lb ft to the front wheels. Two-motor EV-mode means both motors are engaged thus reducing their overall speed and therefore increasing efficiency and reducing battery drain.

Once the battery charge is exhausted (usually after about 40 miles), the petrol engine starts, which extends the range to about 310 miles. Single-motor extended-range driving is where the petrol engine drives the secondary motor/generator, which supplies current to the primary electric motor to drive the front wheels. At high speeds, the secondary motor works as a generator.

The question is why you’d want to squander top-speed efficiency by using the engine to generate electricity for the motors. In the US, however, the Ampera/Volt was major news, because of how it was sold as a technically different solution to the Toyota Prius hybrid.

What's it like to drive?

While based on a Vauxhall Astra floorpan, the overwhelming impression of the Ampera is just how special and different it is. Passers-by walked up to ask about it, people took photos and oncoming drivers craned their necks so much we feared for their safety.

The aerodynamic styling is distinctive and efficient, partly because the car needs all its overrun inertia to charge the battery rather than heating the brakes.

In the cabin, electronic displays in front of the driver and in the centre console convey a huge amount of information, but can be confusing. In spite of the massive battery pack along the centre line, the driving position is comfortable.

A polyphonic whooshing noise indicates that the Ampera is ready for action. Performance is brisk and fuel consumption impressive, but the ride is soft and the damping occasionally struggles to contain the bouncing moments of the heavy driveline. The steering gives little indication of what the front wheels are up to, although the predominant trait is nose-on understeer. Performance car it isn’t, but it can be driven briskly provided the road is reasonably smooth.

With a total range of about 310 miles, you need to keep your eye on the fuel gauge as well. We had our own form of range anxiety on the Cairnwell Pass when we realised we were fast in danger of running out of fuel, both volts and litres. We only just made it into Braemar to fill up with the latter. For all the press releases that arrive each week telling us about new battery charging points, there are still precious few in the farther flung parts of the country.