Before we go any further, I must pause for a small giggle at the thought that a car with the words Premium and Luxury in its title is not the most sumptuous and expensive in its range. But that's the way Jaguar's naming system works - if you want an XJ that's more premium and luxurious than Premium Luxury, the Portfolio is the car for you.The Portfolio was the first of the new XJs tested by CARkeys (see this article for our views, plus remarks about the XJ's looks, visbility and instrumentation which I won't repeat here). This time round we've gone for the magnificent 275bhp three-litre turbo diesel engine again but picked the standard- rather than long-wheelbase body style.The difference in length is worth several comments on its own. First of all, the LWB car is the one to go for if anyone above-average in both stature and social importance is likely to be carried in the rear. An XJ is generally a comfortable thing to sit in, but the back is none too spacious if you're more than six feet tall.That doesn't entirely seal the deal, though, because there is a marked difference between the ride qualities of XJs which differ only in length. The LWB car I drove previously, like all current Jaguars, suffered from being fitted with absurdly low-profile tyres, but in other respects it rode rather well. This one, not so much. Driven on the same roads at the same speeds as the last one, it bucked alarmingly over sudden crests, and several times sent my head crashing into the (fortunately very well-upholstered) roof.This is definitely a suspension problem, yet the same set-up allows for very sporty behaviour round corners. Of course it's nimbler than the LWB car, but the effect is greater than I was expecting. Strange as it may seem, the standard-length XJ is not well-controlled if the suspension is simply trying to control upward and downward movements, but suddenly behaves supremely well as soon as an element of lean is introduced.The other outstanding thing about the XJ is the way the major controls work. There is no lack of precision, but at the same time the very first reaction to any input of acceleration, braking or steering is industry-leadingly smooth, and you would have to have the physical co-ordination of a drunken orang-utan to make the car proceed in a jerky manner.In some ways (and we're back to the looks, visibility and instrumentation again) I feel the XJ shows that Jaguar has failed to maintain standards it achieved with the previous model, but when it comes to smooth operation this is one of the finest cars on the market. Engine 2993 cc, 6 cylinders Power 275 bhp @4000 rpm Torque 443 ib/ft @2000 rpm Transmission 6 speed auto Fuel/CO2 40.1 mpg / 184 g/km Acceleration 0-62mph: 6.4sec Top speed 155 mph Price From £58431.00 approx Release date 01/01/2010