Taking the excellent Coupe as the starting point, the roof has been lopped off and in its place is a triple-layer fabric hood with a special insulating foam sandwich. BMW has eschewed the current trend for folding metal roofs; they are too heavy, too expensive and, crucially, take up too much room in the boot, which is not ideal when one of the key selling points of your vehicle is the fact that you can carry two sets of golf clubs in the rear.

With the roof in place, the Convertible manages to echo the lines of the Coupe, thanks in part to the Jaguar XJS-style buttresses at the rear, which extend the roof line further back towards the boot.

This unusual arrangement houses an electrically-powered rear window. At the touch of a button it lowers to give you a draft-free connection with the outside world. What it also means is that more of that superb, burbling V8 exhaust note permeates through the interior.

But with the window up you could almost be in the Coupe, such is the quality of the conversion. With no framework visible and no additional wind noise over the Coupe, this is a mightily impressive soft-top. You can even operate the fully-electric roof at speeds of up to 20mph, saving you from a red-faced moment when the traffic lights turn green and you're halfway through putting the roof up.

All very impressive, as is interior space. Usually when a folding roof is added to a car, rear seat space suffers, but the 645 makes a decent stab at being a proper four-seater; a point proved when I persuaded a 6ft 4in friend to sit in the back, with the roof up, while his wife sat in the front. Amazingly, he fitted in quite well. It would certainly be adequate for an occasional trip down to the pub (or perhaps that should be to the golf club).

That's the good news then; now the bad. While lopping off the roof, BMW has had to massively reinforce the body to replace some of the chassis stiffness lost in the conversion. Unbelievably, the kerb weight of this Convertible is a full 200kg greater than the 645 Coupe. Now that's a lot of extra bulk and even with the deceptively powerful 4.4-litre 333bhp V8 fitted it struggles to cope with the massive weight, losing most of the sparkle that the Coupe so effortlessly seems to deliver.

The added weight affects everything from acceleration and handling to braking. Perhaps if you hadn't driven the Coupe it wouldn't matter quite so much but, if you have, the negative effects of the additional bulk are inescapable.

It's a surprise, then, that even with this much extra reinforcement going into the car there's still the occasional shudder transmitted through the cabin when blatting down a lumpy layer of tarmac, especially with the roof down. Things improve markedly with the roof up, but that's hardly the point with this car is it?

I was also surprised by how drafty this car is with the roof down (and I know that sounds silly, but rivals manage to protect their occupants from wind buffeting much better). BMW makes an extra draft excluder that slots behind the front seats to counter this but the trouble is that once in place you've got zero access to the rear seats and I reckon that would be a right pain to live with on a day-to-day basis.

I admit I'm not the biggest fan of open-topped four-seater cars, unless they massively change the character of the base car for the better (like perhaps the Saab 9-3 or Audi cabrio did).

This chopped 645 hasn't changed my opinion: it'll need more than a clever rear window to recommend it when the Coupe is so good in the first place. But then again, people buy these cars to get noticed, and the 6-series Convertible will certainly achieve that. Whether the chop actually improves the car is another matter.