There’s a curious appeal to testing a prototype car.

One of the challenges of reviewing a production vehicle is to achieve a balance between the car’s mechanical characteristics, its looks, the build quality and finish, the driver friendliness, the space, the visibility and some finger-in-the-wind, gut-instinct assessment of overall appeal compared with its competitors… and probably a few other things that only spring to mind when you start writing it.

With a prototype, however, as I was able to remind myself on Wednesday when I drove the Focus BEV on the test track at Ford’s Dunton Technical Centre, there’s an extra level of freedom.

We’ve seen enough Focuses – it has been Britain’s best-selling car, after all – to have a good idea of the build quality and finish of the car… and the interior design… and the space… and the looks… and so on.

So the challenge is talk about the thing that makes the BEV different from other Focuses, which is of course its zero local-emissions drivetrain.

This reminds me of a big reason that Ford has chosen Hillingdon, a London borough otherwise neglected by the media, for its public trials. Scottish and Southern Energy supplies the electricity there, and their power is hydroelectric, from all that falling water in Scotland.