Some cars take a while to grow on you. You have to drive them for a few hundred miles to realise how good they are. Other cars you fall in love with immediately but within a day that love's gone cold.The Jaguar XJ LWB saloon is neither of these. It makes you smile as you're walking towards it for the first time, and you're still smiling hundreds of miles later.It's an achingly lovely car to look at from most angles. Unlike many long-wheelbase luxury or executive models, this one not only retains the sleek and sporting lines of the standard model, it actually enhances them by slightly drawing out the curves and slopes.The 20" wheels fill the gently flaring wheelarches and are perfectly proportioned to the glass and metal of the flank. It's a rounded bullet of a car with narrow headlights either side of a gaping grille that hints at the hidden engineering behind it. Bodywork fold lines are subtle and the gently landscaped humps here and there emphasise its sporting character.The only external feature I still have difficulty appreciating visually is the upright rear light clusters. They just aren't as sporty-looking as the rest of the car.This is the high-spec Portfolio model with the three-litre V6 turbo diesel engine which pumps up to 271bhp through an eight-speed automatic gearbox to the rear wheels. It takes just 6.4 seconds to leap from a standstill to 62mph and reaches the limiter at 155mph. Amazingly, its official average fuel consumption is almost 45mpg. It would cost £70,975 in standard form, but with the extras on this one, the price on the tag is now £86,460.It's actually only about five inches longer than the standard car, and inside there's only an extra four inches of legroom so it's far from a stretched limo. It weighs around 172lb more and the turning circle is only about a foot and a half wider so it's not exactly a radical redesign.Although the back seats are clearly as important as the front, it's a faultless driver's car. It's responsive and lithe on the open road, and the only time it feels big is when you're on narrow country lanes or trying to park it in the drive or garage.First impressions are great. Keyless entry unlocks the doors as you tug at the handle. The door mirrors unfold as you get into the leather seats and the start/stop button pulses red like a heartbeat. Stab at it and the big cat comes to life.The 18-direction electronic adjustable seat moves you into your perfect driving position and the steering wheel comes down to meet your hands. The knurled and chromed gear selector rises out of the tunnel and after fastening your seatbelt you're good to go. Select Drive, drop the electric parking brake and the world beyond the bonnet is yours.On the road, the only sign that you're changing gear is the twitch on the revcounter. The engine is smooth and quiet and the gearshifts are virtually imperceptible. I find the suspension set up to be the perfect balance of support and pliancy, transmitting only the worst excesses of a failing road surface and supporting me through a spirited dash along the snaking country roads. I find the steering a bit light at motorway speeds, but it helps keep you alert as the car seems to be doing everything else for you.The transmission has a Sport setting which holds onto the gears for longer to get a better response, but the car leaps out pretty smartly in Drive if you need to stamp down quickly. Selecting Sport however makes things like a sprint start or overtaking manoeuvre much more of a premeditated act of violence.Rear visibility, even on the open road, never mind while reversing, feels seriously compromised. The back window has a lot of glass, but it's steeply sloped so the view of what's behind you is actually very small. You'll barely see the headlamps of the car that's following – just its windows and roof.The headrests on the back seats squeeze the view even more. Safe reversing or kerbside parking depend utterly on a rear view camera (useful but mounted a little too low to the ground to give good perspective) and proximity alarms front and back.The internal decor is almost perfect. This car has heated/cooled leather seats with contrasting white piping and stitching. The dash, steering wheel and pretty well everything that isn't metal is also clad in cow. Accentuated detail is in high-gloss black to match the external paint. The split-level dashboard which sweeps a long, curved amphitheatre around under the windscreen helps create the impression of space. Look carefully under the instrument cowl and you'll see the "dials" are not actually there. They're images on a built in screen, but they look real.I have a slight problem with the phosphor-blue mood lighting in there at night. It's reminiscent of the bar in an airport hotel.Nice touches include the standard-fit twin sunroofs (only the front one opens). Bad touches include the manual gearshift paddles under the steering wheel which feel like exceptionally cheap injection-moulded plastic. When everything else in there exudes such luxury, why can't we have aluminium? Even Lego would feel more substantial.My other main gripe is with the satellite navigation. Jaguar says it's been upgraded, but in my experience it's just incompetent. I went through the settings to allow any kind of road to be chosen to navigate from southeast London to Cornwall. I checked the short route option, then the fast route option which was expected to be three hours quicker. But my own route, worked out on a map, got me there another hour sooner.After the ride and performance though, perhaps the most important part of the XJ LWB though is the rear-seat accommodation. The test car is equipped with the Premium Rear Seat package (£8490) which has two individual heated and cooled tilting seats that have memory options for the position. Aircraft-like tables fold up and forward from the backs of the front seats and there are TV screens mounted above them - rather like bolt-on afterthoughts, to be honest.There are left and right climate controls, headphones, that second sunroof and tinted privacy glass all round. Other than a flight attendant and a fridge stocked with champagne, I can't think what else you could need or want in there.Slightly disappointing is the size of the boot, and its entrance is narrowed a bit by the shape of the car's tail end.I've taken this car the thick end of 700 miles around Britain's roads from the Midlands, down into central London, across into Oxfordshire and ultimately into Cornwall. It's been an extraordinarily comfortable and relaxing experience, and I've been proud to be seen in it, as have all the people I've taken out in it.I've discovered a good game to play with it too. If you open the door for your passenger and slip in the odd "Ma'am" or "Sir", then surreptitiously look at bystanders' reactions, it's hilarious. They clearly think you're ferrying round some minor royal, or the head of MI6. Engine 2993cc, 6 cylinders Power 271bhp Transmission 8-speed automatic Fuel/CO2 44.8mpg / 167g/km Acceleration 0-62mph: 6.4 seconds Top speed 155mph Price £70,975 Details correct at publication date