BUYING most of BMW's range involved a healthy dose of heart as well as head - they are mostly cars aimed at people who love driving.

Well, here's one that might make you phone your accountant before signing on the dotted line. He might then tell you to choose a different model.

The big X5 is now available with an electric motor to assist the petrol engine and save a bit of fuel in the right circumstances.

But it might not, and you would then be better off buying the diesel, which costs only £215 more, goes faster and might actually save you money on longer journeys.

But here's where the accountant comes in; because the new X5 puts out such tiny amounts of tailpipe emissions in the official tests it qualifies for zero road tax and - more important for all the business people who fancy one - there's a £2,711 saving in benefit in kind each year.

Annoyingly, the emissions are a tiny bit too high (by a measly 2g/km) for the car to dodge the London congestion charge and the same minute tailpipe excess means the car does not qualify for a government plug-in car grant, which would lop £5,000 off the price now and £2,500 off from March, when the rules tighten.

Topping a dead battery to full takes less than four hours from a domestic plug, about an hour less if you have a BMW Wallbox installed. The car can then be ordered to mix petrol and current in the most economical way, save all its battery for later or combine the two power sources for maximum urge.

Of which there is plenty. And so back to the heart; where you find a car that looks precisely like the diesel equivalent with some modestly different badging and a flap in a front wing where you plug in the charging lead.

Even lumbered with the extra weight of a biggish battery (which robs the boot of a little room under the floor and means you can't have this X5 with a third row of seats) this is a car that flies when you prod both power sources into action - less than six seconds to sixty is brisk in anyone's book.

Driven like that, you'll exhaust the battery in minutes. With a little restraint you can drive up to 40mph-ish on flat roads on electrical power alone and get somewhere like the 19 miles a full charge can offer.

BMW reckons 80 per cent of journeys with an X5 are less than 19 miles (station car park and back, anyone?) and used like that your travelling expenses will be negligible.

With commendable honesty, BMW then adds that a 40 mile daily round trip should return up to 47mpg, but stray into longer distance territory (125 miles and up) and economy tumbles to 27mpg.

That's more economical than the petrol X5 with similar performance, says its maker; perhaps missing the point that the 3.0 litre diesel would do better. Which is why buyers go for it, of course. In petrol-focused China and the States the argument for the X5 hybrid makes more compelling sense.

It remains a decent drive in any case; a car built seemingly from a piece of solid steel, so tough does it feel. It's big, but you soon learn not to fear multi-storey car park ramps and Waitrose parking bays; sitting tall helps here.