Updates for the 2013 model include a new eight-speed automatic gearbox and an engine stop-start system, which help cut CO2 emissions by as much as 14%.
A new supercharged 3.0-litre V6 petrol engine that will be used in Jaguar's forthcoming F-type sports car also joins the range, replacing the 5.0-litre V8.
What's the 2013 Jaguar XJ like to drive?
The new supercharged engine produces 335bhp only 45bhp less than the old V8.
Things are a bit flat below 3500rpm, but get the revs above 5000rpm and the XJ absolutely flies hitting 62mph in just 5.9 seconds.
All XJs now come with a new eight-speed automatic gearbox. Shifts are generally smooth, although with the new supercharged engine, the box is a bit too eager to change down a gear when you just want to accelerate slowly and smoothly.
The 2013 XJ gets a new eight-speed auto and a stop-start system, which help cut emissions by up to 14%
This is less of a problem in the 3.0-litre diesel, because its maximum torque is developed at much lower revs.
There are plenty of other reasons to choose the diesel over the petrol: its 5000 cheaper, almost as quick in the real world and is much more economical, averaging a claimed 47mpg compared with the petrol's 30mpg.
The lightweight aluminium construction means its lighter than its rivals which, combined with good body control, makes the XJ remarkably nimble for something so big.
The drawback is a firm low-speed ride, but things do at least smooth out on faster roads.
What's the 2013 Jaguar XJ like inside?
Nothing much has changed here. The lavish veneers, old-school bulls-eye air vents, and ebony and chrome detailing punch home the old-money message.
These traditional ingredients are complemented by cool mood lighting, Jaguars signature rising rotary gear selector, digital instrument dials and a touch-screen infotainment system.
Sadly, although the touch-screen looks the part, its functionality is less impressive, because the icons are quite small, while the responses of the system are frustratingly slow.
Theres loads of room in the front, but things arent quite so impressive in the back, because although you sit very low the sloping roofline still eats into headroom.
Even the standard car has plenty of legroom, though, and if you want more, theres a long-wheelbase version.
All XJs are lavishly equipped, with leather-trimmed electric front seats, dual-zone climate control, twin glass sunroofs and satellite-navigation fitted as standard.
A new 20-speaker Meridian stereo system has been introduced to the 2013 XJ, which is standard on Portfolio models and an option on other trims.
Should I buy one?
If youre in the market for a luxury car and you plan to drive it yourself, then the XJ deserves serious consideration. Its so much more agile than any of its rivals, yet is still comfortable and refined enough to cosset on long motorway jaunts.
However, if youre going to lounge in the back while the hired help takes the wheel, youll probably appreciate the greater comfort provided by the Mercedes S-Class.
What Car? says..
Pete Tullin and Will Nightingale