Tuesday: I don't care what anyone else thinks - the basic shape of the XJ may date back nearly forty years, but I don't care because to me it's one of the motor industry's great designs, and I love it. I wish this particular car wasn't silver, though. As far as I'm concerned, XJs are British Racing Green and that's all there is to it. But I'm not going to let colour prejudice stand in the way of my enjoyment of this splendid car.Not that it's without its problems. The last person who drove the test XJ was considerably shorter than I am, and I need to adjust the seat, steering wheel, mirrors and pedals. (Note: why don't more manufacturers make their pedals adjustable?) The only snag is that the rear part of the seat bench can't be lowered. There's a button for this purpose, but when I press it the only result is a disapproving click, so for the first day of this test I feel like I'm driving the car standing up. This is not a great start.Rather more positively, after 336 miles on the motorway the fuel gauge tells me that I've used only slightly more than half a tank of diesel. It must be optimistic. Surely I'm not going to be able to drive 600 miles without refuelling?Wednesday: I won't find out today because I put a mere 64 miles on the clock. Plenty of good news, though. The seat adjustment has returned and I can now sit in a position which lets me enjoy the XJ's amazingly comfortable ride. And I get my first chance to test out the satellite navigation, since I have to negotiate a town I've never visited before during the evening rush hour. The display screen is mounted halfway up the centre console, which is a bit too low for convenience, but the system works splendidly.Thursday: A much longer run today. Once I've dialled in the next destination, the satnav tells me it's 250 miles away, and even though I've been behind the wheels of several other cars already - a bit of off-roading, a bit of performance testing on a mountain course - the idea of driving the XJ for four hours or so is a very appealing one.This car is so relaxing, and requires so little effort, that I find it's having a beneficial effect on my driving. My anticipation and observation are better than normal, which is probably just as well because when I turn my head to check if anything is coming up alongside me in the next lane of the motorway I get a face full of centre pillar. It's a source of complaint, but right now I don't really care.Friday: The fuel gauge wasn't kidding me. I pass 700 miles before the warning light comes on, and I eventually refuel at 728.9 miles. 86.2 litres go in, which comes as a surprise since the handbook says the tank holds 85 litres, and according to my own calculations I've averaged 38.4mpg, somewhat less than the 41.4mpg claimed by the trip computer. Still an impressive figure, though.I'm reminded later that another XJ diesel managed 1002 miles on a tankful under test conditions. I can see how that's possible, but the driver must have had to use some pretty specialised techniques to achieve it. The point about my 38.4mpg result is that I managed it without any of that fancy stuff; I haven't been going particularly quickly, but nor have I driven slower than I wanted to. It's the combination of near-40mpg economy with all the XJ's other abilities that impresses me so highly.Saturday-Monday: A couple of hundred miles of local driving, none of it anywhere near a motorway. The steering wheel is beginning to irritate me slightly; part of the rim, including the bit I like to hold, is covered in polished wood-effect plastic, and it can sometimes be a bit slippery. If I had my own XJ diesel (and I'm increasingly of the opinion that I very much want one), I'd specify a more grippy wheel.Tuesday: The first half of the final long run of this test. 250 miles, fantastic roads all the way, with perhaps five minutes' worth of dual-carriageway somewhere in the middle. A talent shared by all XJs is that they seem to shrink to fit the road. The car is so large that you don't believe it can possibly be nimble, and in all truth it's not very sporty (unlike the similarly-sized XJR, which is unbelievably so), but it changes direction very accurately and disguises its bulk remarkably well. The low overall weight - a result of using aluminium to create a car whose looks belie its high-tech construction - contributes greatly both to this and to the impressive economy.I can't help thinking, though, that there's something missing. This car doesn't seem as responsive as the one I drove a few months ago for a short road test. I hope I can solve this one before I have to relinquish temporary ownership.Wednesday: A Mazda PR person asks me what I think of the XJ. "It's brilliant - I really love it," I say. Then I look across at the MX-5 I've just been driving and add, "In fact, if I had one of these and one of those, I'd be very happy." Reporting this conversation later, I'm informed by a critic that I've expressed a preference for an old man's car and a girl's car. Stuff that. I don't really care about image - I'm more interested in what the cars are like to drive, and these two are at the top of my list.Oh, and I discover what I'd missed during yesterday's 250-mile trip. I had left the gear selector in Drive all the way, so that once I'd got the XJ set up for a corner and applied the throttle I then had to wait for it to choose the most appropriate gear. The suspension, being set up more for comfort than for sharp cornering, doesn't respond quickly enough to make this a smooth process. I remember that I changed gears manually during the short test, so I do that during this final journey.What a difference! Brake for the corner, select the right gear, enter the corner, let the car settle and go. Jaguar's J-gate gearshift - which I and many others have been criticising for some time - makes this a slightly laborious process, but the effect on the handling is wonderful. Suddenly the XJ is nearly as impressive through the twists and turns as it is on a long cruise; it will be better still if Jaguar decides to apply the XK's steering wheel-mounted gearchange controls to the big saloon.By devious cunning I have managed to extend this test a little longer than my allotted span. Now, though, it has to go on to the next journalist in the line. He's a very nice man and I hope he enjoys the XJ as much as I have, but I can't help feeling slightly resentful. This, you see, is the car I should have. I deserve it. I'm a good person, admired and respected by all who know me, and I'm kind to animals. When the world sees fit to reward me, it can do so by giving me an XJ diesel. But make it a green one. Engine 2722cc, 6 cylinders Power 204bhp Transmission 5-speed automatic Fuel/CO2 35.0mpg / 214g/km Acceleration 0-62mph: 8.2 seconds Top speed 141mph Price £45,020 Details correct at publication date