Mercedes boldly claims that the Roadster version of the McLaren SLR will have identical performance to the current coupe. That means an astonishing roof-down top speed of 207mph and a 0-62mph time of just 3.8sec, making it the fastest full convertible in series production when it goes on sale in September.

Finding space for the folded roof was always going to be a headache because the SLR is very tightly packaged in coupe form. Although anyone spending in excess of £300K might expect an SL-style folding steel roof, it was impossible to fit one in. Instead Mercedes developed a special fabric roof that retains its structural integrity at high speed but can disappear under an aluminium cover into a shallow cavity created behind the seats. The roof folds away electrically but is not fully automatic – the driver must unlatch it from the screen header before using the button.

Further changes to the car include a revised carbonfibre monocoque chassis to compensate for the lack of roof. The SLR's kerb weight is barely altered though and, according to Mercedes, delivers world-beating torsional rigidity for a topless car.

Prices for the SLR Roadster are yet to be announced but don’t expect much change from £350,000.

Meanwhile over at Porsche the engineers have been busy lopping the top off the 911 Turbo. Purists might not approve but Turbo Cabs are expected to account for a remarkable 40 per cent of sales once the new model hits the showroom in September. Performance of the soft-top Turbo is very similar to that of the fabric-topped SLR, with the tiptronic auto version exactly matching the SLR’s 3.8sec sprint to 62mph. (The manual verion is officially slower at 4sec dead.)

The usual body-strengthening has been carried out and, as on the SLR, there are automatically-activated roll-hoops to protect occupants should the worst happen. The result of all this work is a 70kg increase in kerb weight over the coupe Turbo. Compared with the SLR Roadster, the 911 Turbo Cabriolet is a bit of a bargain at £106,180.