Mercedes will not sell many crossover R-Class people carriers, but a trend is being set for others to follow. Big, bulky and as spacious as an aircraft hangar, it's hardly a thing of beauty, but the R-Class is impeccably built and on the cusp of an entirely new sector.The UK market is expected to consume no more than 2500 in a year, and of them 70% will be long-wheelbase versions, with 60% powered by the outstanding three-litre turbo diesel. All R-Class models have 4Matic four-wheel drive and 7G-Tronic seven-speed automatic gearboxes. It's a match made in heaven and creates a six-seat luxury express which is part MPV, part 4x4 performance coupé and part estate.The target audience are those trendies who naturally aspire to a large 4x4, but need something more people-orientated and spacious, with a touch of elegant practicality and the cache of four-wheel-drive. And the model they are likely to lust after most is the R 320 CDI L with its long wheelbase and price tag of £39,975 in entry-level guise - but £42,925 with the SE pack on my test vehicle.If ever there was a "star choice" model it has to be the R 320 CDI. The powerful three-litre V6 is a superb oil burner - smooth, quiet and good for an average 31mpg. On the road it handles the long-wheelbase R's bulk with little strain. There's a gruff engine note under hard acceleration, but at all other times the 224bhp unit with its 376lb/ft of torque is subdued and flexible. Maximum pulling power is available from just 1600rpm and peaks at 2800rpm - that gives the four-wheel drive monster brisk overtaking ability and an impressive 8.8 seconds for the 0-62mph sprint.The R-Class is built on the new ML chassis. In standard guise it's already a mighty 4.9 metres long but my L model has both wheelbase and body extensions to bring it up to 5.1 metres. It delivers excellent interior space with plenty of legroom for all three rows, but at the expense of parking ease. Light power assistance and parking aids aside, this is one big mamma to slip into a standard supermarket space.With the second and third pairs of seats folded neatly into the floor in seconds there's a huge cargo floor which helps the long-wheelbase R 320 CDI carry a payload of 655kg. From the tailgate to the back of the front seats there's 2.2 metres and a massive 2366 litres of space. Leave the middle row in place and there is still 1048 litres for cargo.The rear seats are close together and best suited for small adults or teenagers. But access is easy in the L model as the middle pair fold and kneel elegantly to allow you to climb aboard.According to Mercedes the standard specification is "generous". That's a matter of opinion. It certainly has substantial safety features with Pre-Safe and Neck-Pro front passenger protection systems, front, side and full length window airbags, ISOFIX mountings with top tethers on all four rear seats, Brake Assist, electronic traction control (4ETS), electronic stabilisation programme (ESP), acceleration skid control (ASR) and powerful ABS braking. And every R-Class gets 7G-Tronic auto transmission, 4Matic 4WD, cruise control, auto lights and wipers, trip computer and partial electric seats.But start adding some extras and your bill quickly mounts. Bi-xenon active lighting costs £1100, full-colour COMAND sat-nav and CD £1760, a rear seat entertainment system £2080 and an AMG styling pack another £2320.The test car was in SE trim - a £2950 upgrade. The same money will also get you a Sport specification. The SE pack adds metallic paint, chrome trim, fully electric front seats, burr walnut trim, full leather and an interior lighting pack with a calming soft orange glow.On the road the R 320 CDI SEL handles well - a tribute to its impressive electronic driving safety aids. Long wheelbase and wide track linked to AirMatic air suspension - standard on the R 500 and an £880 option on the rest of the range - delivers excellent ride quality. Air suspension is carried over from other premium Mercedes products and delivers impressive ride height variation and enhanced comfort.Throughout my test it rained mercilessly. It took me some time to master the car's air-conditioning system to prevent window misting and hitting large pools of roadside water snatched at the large 19" front wheels. It may have been down to the choice of tyres, but the pull to the left was noticeable - surprising for a four-wheel drive vehicle with a 2.2 tonne kerb weight.But there are no doubts about grip. The R-Class is very surefooted thanks to immense grip from the 4Matic permanent four-wheel-drive system. It can be hustled along twisty B-class roads, but you have to constantly remind yourself that this is a very large people carrier. It stands 1.6 metres tall and nearly 2.2 metres wide to the extremities of its large door mirrors.The driving position is comfortable with plenty of adjustment, and the instrumentation and controls follow the encouraging trend of recent Mercedes back to the high ground of top quality. Visibility is good to the front and sides, but the plunging window line at the D pillar and a relatively narrow tailgate screen hampers rearward vision.Once again Mercedes' innovation is paying dividends and introducing the car buying public to a new concept. It's working well with the CLS coupé/saloon. But the impressive R-Class is a lower-key vehicle which, I suspect, will never be seen in large numbers on British roads. Engine 2987cc, 6 cylinders Power 224bhp Transmission 7-speed automatic Fuel/CO2 30.4mpg / 253g/km Acceleration 0-62mph: 8.8 seconds Top speed 134mph Price £42,925 Details correct at publication date