EIGHTEEN is the magic number for the latest model to join the MINI clan.

That's the number of seconds it takes for the new MINI Convertible to transform itself into Britain's coolest open top four-seater.

It's also the maximum speed at which the hood can be operated when on the move.

The new MINI Convertible is arriving just in time for spring and is set to continue the trend which has seen it become the nation's favourite cabrio.

Since the first version hit the scene five years ago almost 30,000 have been sold in the UK making it our top-selling convertible.

Built in Britain at the BMW-owned MINI factory in Oxford the car is the drop top take of the new hatchback MINI launched last year.

It is four inches longer than the previous MINI Convertible and that has helped it grow inside with slightly more space in the back and in the boot.

Leg room has increased by almost 20 per cent and at 215 litres it's now got a fair sized boot for a baby convertible.

Roof down - and the hood folds on to the back of the car - and space falls to 160 litres, enough space for a couple of overnight bags and a bit more but roof up and there's a clever opening mechanism allowing for larger loads.

With the boot lid opening downwards - and it's sturdy enough to create a platform that can hold 80 kilos - the top of the luggage compartment can be raised to accommodate bigger objects.

It's a practical feature and one of several that will keep the new MINI top of the open tops.

Realistically the new car is likely to be used as a two seater and a wind deflector attached over the rear seats to keep down noise and buffeting when driving fully exposed to the elements.

Even at motorway speeds it is possible to hold a conversation at normal levels although you may have to crank up the volume on the sat nav to hear instructions clearly when driving topless.

The roof retraction is much quieter than before thanks to an upgrade to the electric motor and that in itself helps create a quality feel about the new MINI.

The roll over protection devices have now been incorporated into the body of the car and with the roof down it's clean cut and much more stylish.

With the hood up it is hard to spot the changes although the convertible is instantly recognisable as a new generation MINI.

More potent Cooper S models have added road presence with aerodynamic front bumpers, a mesh grille complete with S motif and the now characteristic air scoop on the bonnet. It also gets twin exhausts set centrally under the back bumper.

The S versions look more planted and come with sportier suspension and sharper handling than the basic Cooper convertible.

We have just tried out both Cooper and Cooper S models, priced from £18,475 and £22,430 and it is the more expensive Cooper S which suits best.

There's more bite, more grunt and more thrill from the Cooper S but there also more thirst.

MINI claims the 2.0-litre 192bhp turbo engine in the Cooper S is good for 46.3mpg with emissions of 139g/km while it can knock off 0 to 60 in 7.2 seconds.

It feels - and is - noticeably more powerful than the 1.5-litre three-cylinder engine used in the standard Cooper.

However, both cars we tried had an uncharacteristically stiff action to their six-speed manual gearboxes and rearward vision through the mirrors and relatively small rear screen remained compromised.

Fuel consumption on similar runs was an average of 38mppg for the Cooper S and 43 for the Cooper (officially rated at 55.4mpg and 114g/km) which is nothing to grumble about.

A diesel will be added to the range soon and so will a more powerful John Cooper Works model and those will cost from £20,225 and £26,230.

Inside, the treatment on the convertible is the same as in any other of the new breed of MINI and although the drop top is fitted with a rear view camera and parking sensors as standard there are plenty of options to be had.

Kitted out with sat nav, full connectivity and multimedia hook ups that will be musts for most MINI fans and Cooper we drove was close to £24,000 while the Cooper S was pushing £30,000 - premium prices for premium cars.

New to the options list, and for those who want to fly the flag for Britain, is a Union Jack pattern woven into the hood - a feature following on from the popular red, white and blue roof top decal on the hatchback.

It's a £450 option and only in a monotone design but shows that the car is true to its roots. MINI says no other national symbols will be available - possibly a wise move with the forthcoming EU referendum looming.