I should be knocked out by this car, but I’m not. If you had told me about a 333bhp BMW that would hit 60mph in less than six seconds, handle as few sports cars do, yet is a genuine estate, you’d be close to defining my idea of a car for all reasons.

But in this BMW 545i Touring, something has gone wrong in the execution.

I’ll deal with the good bits first. Like so many cars, this estate version of the 5-series is better looking than the saloon. And, as its lofty power output promises, the 545i is quick in a straight line and awesome in the bends. Equipped with the £1,550 Dynamic Drive option — the car I drove came with more than a VW Polo’s worth of extras over its £44,375 list price — which all but eliminates body roll, you’d need a Porsche Boxster to stay with this estate down a winding road.

Had the 545i Touring done this while performing its first duty as an estate car, you’d see at least four stars at the end of this test. But this car is about as convincing as an estate as Clarkson would be a spokesman for the Caravan Club.

Right down at the fundamental end of things, it hasn’t got a very big boot. With seats in position it will stow 500 litres of luggage; seats down this rises to 1,650 litres. The figures for its closest rival, the Mercedes E-class estate, are 650 and 1,910 litres.

Then there’s the high boot floor, meaning that anything you want to put into the back has to be lifted a foot further off the ground than with the Mercedes. If you have a dodgy back or a new refrigerator to get home, this is a serious consideration. Finally, the rear seats don’t fold completely flat, meaning you have to push large heavy items slightly uphill before they’ll fit in.

Now we’re used to BMW selling estate cars that barely do justice to the word and perhaps that’s why they’re called Touring. But down at junior executive level, where the 3-series Touring rightly remains the most coveted car in its class, these cars are not bought for their carrying capacity. They are “lifestyle choices”, bought to make owners look cool.

But I don’t see this happening at 5-series level. No one ever looked cool driving a large estate. These cars are bought by older people to carry the kids’ mountain bikes, take hedge trimmings to the tip and collect the furniture granny won’t need any more. In short, they need the space.

Then there’s the ride. On our bumpy back roads the suspension serves up a ride quality you’d find hard to forgive in a sports car. The blame, apparently, lies with its run-flat tyres, which have to have bomb-proof sidewalls. But given that you are likely to have a puncture less than once in every 100,000 miles and that a space-saver spare is a £70 option, I can’t see the point.

However, we’re getting used to BMW’s love of technology for its own sake. Its iDrive control system, though improved, still infuriates. This 545i was also equipped with active steering, which varies the gearing of the steering according to speed. It entails less wheel-twirling in town and more stability on the motorway, but BMWs have never been deficient in either area. What it means is that, at 5-30mph around town, you’re never quite sure how the car will react to any given steering input.

Of course on smooth, twisting open roads, the 545i Touring is a thing of wonder, but the price paid is simply too much. It is ridiculous that you should have to seek out such an environment before an estate car will put its best foot forward.

If you’re going to buy a 5-series Touring, save more than £10,000, get a diesel 530d Touring and do not be tempted by active steering. I drove one after the 545i and was astonished by its performance (it’ll still do 150mph), economy and flexibility.

Better still, buy a Mercedes E-class estate. In saloon form the BMW comfortably outpoints the Merc, but as an estate the E-class’s fine ride, huge boot, flat floor and low loading lip makes it a clear winner. This time Mercedes has thought and tried harder and, all over the car, it shows.

VITAL STATISTICS

Model BMW 545i Touring
Engine type V8, 4398cc
Power/torque 333bhp @ 6100rpm, 332 lb ft @ 3600rpm
Transmission Six-speed automatic (£1,350 option)
Fuel/CO2 25.2mpg (combined) / 271g/km
Performance 0-62mph: 6.0sec / top speed 155mph
Price £44,375
Verdict Curiously, a truly disappointing BMW

THE OPPOSITION

Model Mercedes-Benz E500 Avantgarde Estate, £45,960
For Colossal boot, superlative ride, excellent refinement
Against Strange driving position, not exactly a riot to drive

Model Audi S6 Quattro, £49,525
For Strong performance, great looks, excellent build quality
Against Rather dated now and has lumpy ride; soon to be replaced