Mercedes has a good recent record launching niches. The CLS four-door coupe no-one knew they wanted has sold well with a 2112 total Jan-June 2006, of which 445 went to fleet. Now we have the R-class: a car so genre-bending that grown-ups in this office have argued for more than five minutes about what it is: luxury 4x4, estate, MPV?

Merc calls it a "Luxury Touring SUV" which covers all but our MPV option and it's a fair shout. It's a high-roofed big estate with six adult seats and permanent 4x4. Since its April UK on-sale date to the end of June it has sold 367 total (180 fleet) but from September it will be joined by two new models to top and tail the range and improve options. The 510PS 6.3-litre petrol R63 AMG version sits up top at £74,115 but will account for only a handful of extra sales, but the new entry-level R280 CDI should give fleets another reason to look at this niche.

The 190PS 3.0-litre V6 diesel has already been slotted into the E- and ML-class and been generally well-received. It's good in the R-class too. Despite the R being heavier than both the E- and even ML-class, it still pulls well and easily from standstill in conjunction with the smooth semi-auto gearbox with two people aboard. With six adults it will have to work harder but at least there won't be much luggage to factor in - this engine variant only comes in a short wheelbase version for the UK so there's not much space for bags. The SWB version offers 244 litres 'seats up' versus 314 in the LWB versions (but the biggest difference between the two is in greater rear legroom).

The gears can also be worked by buttons behind the steering wheel which now work more logically than previous Mercs - right for 'up', left for 'down' - and help keep control of the car on twisty roads. The driving modes - normal, comfort and sport - also help, although after much button pressing 'normal' seems adequate for most driving conditions.

With 30.4mpg and a likely top 35% CO2 tax band identical to the slightly more powerful 221PS 320 CDI, the main reason to choose a R280CDI is its £2000 lower P11D.

Merc is still coy about R sales targets but reckons the 280 and 320CDI will be the top sellers. To that end it's a useful addition to an interesting if not fully-formed niche.

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