IT'S hard for car makers not to keep adding bits to their cars in the hope that more proves to be better with buyers.

The latest version of the world's most successful sports car - the Mazda MX-5, as if you didn't know - is about as added-to as you can get. And the buyers, or lots of them, will love it.

A quick glance at the new car gives all away, its sweeping, solid roof a very obvious style statement that this car is different to the more classically proportioned soft top roadster.

It will, however, let the sun shine in almost as fast as the older car, in mere seconds and at a brisk walking pace if you don't want to stop. If only the once, it's worth asking someone to lower the roof as you stand alongside and admire a finely choreographed performance.

Distant whirring from electric motors pulls the prominent rear buttresses upward, allowing the roof panel to vanish into an interior darkness before the rear flattens itself again and the car signals its readiness to move off.

In the conversion process the RF version has added a modest 45kgs over the fabric-topped MX-5 but lost nothing in boot space (eat your heart out Jaguar XK), which means there is enough room for a couple of squashy overnight bags and still space to spare for those bits of travel detritus that go with you always.

Inside the cockpit you will notice the continuing absence of a glove box or door pockets, (lack of) features shared with all versions of the new MX-5, but there is a small lidded compartment on the bulkhead between the seats, two smaller compartments hidden by the seat backs themselves and a useful lidded cubby behind the gear lever.

So, the MX-5 is probably as practical a two-seater sports car as you could hope for. That has been a given for the 28 years the car has been on sale in one form or another, of course, and a major reason it has sold more than a million examples, some 120,000 of them in the UK.

The folding hardtop arrived late in this story line but is proving popular, to the extent that eight in ten final orders for the old version were for the hardtop. That figure might soften a bit but Mazda must be happy its investment in the technology is going to pay off.

It will. For starters you'll pay a £2,000 privilege for the RF (for Retractable Fastback) version. Prices start at £22,195 for the 1.5 SE-L Nav and rise to £28,999 for a 500-off 2.0 Launch Edition with fancy Recaro seats, special alloy wheels, metallic paint and glossy black roof, spoiler and door mirrors.

Why will buyers bother with any of the RF versions? Perhaps for the perceived added security of a solid barrier between the outside world and your valuables, even if nobody made targets of the existing soft top.

Or perhaps they simply like the newcomer's looks. To these eyes it lacks the honed lightness that Mazda has tried so hard to recapture in this Mk IV MX-5 but it certainly looks stand-out different, which may be a clincher in the buying decision.

The transformation has been thoroughly done, with subtle changes to the suspension and body bracing, along with extra sound deadening in areas like rear wheel arches.

This latter feature and the new top means a quieter cabin, according to Mazda's measurements. Well, it remains very much a sporting machine decibel-wise, making enough noise at a motorway cruise to mean raised voices and a tweak of the radio volume control. All three cars we tried also had noticeable and surprising wind roar from around the passenger side door window, but much less for the driver.

Roof down and a winter sun helping raise the emotional temperature of driver and passenger, the RF remained tolerably civilised even at enthusiastic cruising speeds and let you enjoy the traditional delights of open top motoring, from smelling nature in the raw (cows in fields, notably) to enjoying the rasp from the Mazda's exhaust.

Producing that noise is either a 1.5 litre 131 horsepower engine or a 160 hp 2.0 litre. The tiny weight penalty of the fancy new roof has had no effect on performance, either 1.5 version managing 127mph, the dash to 62mph in 8.3 seconds and an identical 47.1mpg average fuel consumption. The heavier car is slightly more polluting, posting 142g/km against the 139g/km of the roadster.

The 2.0 litre engined cars are also identical, even this time down to their tailpipe emissions. Figures of 133mph/7.3 seconds/40.9mpg and 161g/km translate on the road into a car that needs rather fewer gearchanges when the going turns brisk.

Although you might not mind ratio swapping with a gearbox this good; there can't be a car on sale today with a more satisfying gearchange. The smaller engined version also feels a bit lighter on its feet - more properly sports car, if you like.

Both of them managed better fuel figures, even driven hard, than their official ratings. At 43mpg for the smaller engine and 42mpg with the 2.0 litre, these are not going to cost a fortune to run. Insurance groups between 26 and 29 are not quite such good news.

Still, the best news is that there is now a choice of MX-5 models to tempt you, from basic but delightful soft top to a little less 'authentic' but still captivating new RF. Enjoy making the choice...