When I was a kid, I had a friend who used to collect things from a nearby army training ground. Spent machine gun rounds, the occasional flare, that kind of thing.

That meant he spent most of his time looking like some kind of well tooled-up magpie, but his rather questionable predilection for spent artillery required an increasingly inventive selection of places to hide his semi-lethal trinkets from his parents who, if they’d known, would have reached for the yellow pages faster than you could say ‘bomb disposal squad’.

I have a suspicion my friend now works for Jaguar, because someone in their engineering department clearly felt the best place to hide a weapons-grade supercharged 5.0-litre V8 was under the bonnet of an otherwise innocuous-looking estate car.

From the outside, the XFR-S Sportbrake offers few clues that behind the enlarged air intakes lies a lump of unexploded ordnance. Specify a muted exterior colour such as a classy metallic grey, and even a passing NATO weapons inspector would be hard pressed to tell the R-S apart from a diesel XF with a few tasty options.

Those in the know, however, will readily spot the 380mm brake discs and red calipers peeking out from behind the 20-inch alloy wheels, while the modest carbon fibre additions to the front and rear bumpers join with the quad-pipe exhaust system to give more of a clue as to what lies beneath.

The clues mount up once you climb inside, particularly once you clock the desperately appealing contrast piping and stitching and the equally attractive carbon weave material on the seats and doors.

Beyond a couple of modest R-S logos, the rest of the cabin is largely XF business as usual, bar the addition of a rather special 825W Meridian surround sound system. It’s loud enough to set off land mines within a 50 foot radius and remains deliciously distortion-free even at ear drum-perforating volume levels, although to our abused ears the Mark Levinson system employed by Lexus offers slightly tighter bass control.

Rear seat passengers will find the Sportbrake offers slightly more headroom than its saloon brethren, although whoever draws the short straw and finds themselves in the middle seat will have to contend with the high transmission tunnel.

Cargo space is a useful if not desperately commodious 550 litres, increasing to 1,675 litres with the rear seats folded.  There’s a handy selection of load hooks and rails to help keep your belongings restrained, too.  Just as well, as you’re going to need them.

With the practicalities out of the way, it’s time to talk about the small invasion force that’s hiding under the bonnet.

Lift the lid and you’ll find yourself staring at a supercharged 5.0-litre V8 that rumbles away to the tune of 542hp and 680Nm of torque. Mated to an eight-speed ZF automatic transmission, it can thunder its way from 0-60mph in just 4.6 seconds and keep on trucking until its electronically-limited 186mph top speed. Not even Hans Blix could hide this level of performance.

Press the start button and the cat is well and truly propelled out of the proverbial bag as the V8 auto-revs into life with a crackle that sounds like small arms fire. Give the throttle too many prods early in the morning and you might find your neighbours start a campaign to make you the subject of a UN resolution.

Inside the cabin, things are more subdued, however. While the exhaust pops and crackles like a ‘60s racer, particularly on over-run, much of the XF’s vocal performance seems aimed at those following in your sonic wake. Jaguar’s engineers have done an excellent job of tuning the sound of this beast, it’s just that if I’d spent over eighty grand on one, I’d like to be the one getting the benefit from all that work.

There’s no masking the performance available from that engine, however. Plant your foot in the carpet and everything around you instantly becomes history.

The ease with which that happens is what makes the XFR-S so potent, and much of the praise for this should be laid at the feet of the excellent automatic gearbox.  Rarely was there a moment during our time with it that it made a poor choice of gear, but even if it did the speed with which it reacts to a tug on a steering wheel-mounted paddle (complete with an automatic throttle blip) means this could never be an issue anyway.

In Sport mode you get the final say as to which gear you’re in, and I have to say, it’s probably the fastest-responding automatic gearbox I’ve driven, with new ratios engaging practically immediately.

This rapid response helps makes a series of corners even more enjoyable, and it’s here that you’ll discover the handling is more than equal to the task.

Body control is incredibly tight, turn-in is flat and crisp, and its natural state through a corner is remarkably well balanced for a car this size – until you give it an intentional bootful, of course, and hang the rear end out, something that’s tantalisingly easy to do even without engaging dynamic mode.

The XFR-S’s party trick, though, is that this level of performance is all wrapped up in a huge blanket of civility. For all its taught body control, the ride is incredibly comfortable. Whereas the German competition gives things a harder edge that can become tiresome, the Jag gives you genuine all-day comfort and a ride that wouldn’t upset your mother.

Rarely does a car come along that offers not just this level of space and performance, but also a matching degree of refinement and a sense of theatre.

Ok, so it’s not cheap – you’ll need to write a cheque for £82,495 to get one, and it’s got a bit of a drink problem, too, recording an average during our testing of just 19mpg. But it’s a bit like your favourite aunt who drinks too much sherry at Christmas – somehow you just can’t stop yourself from topping up her glass.

Rather than start an arms race with the German competition, Jaguar instead created something a little more refined. Perhaps even tasteful. Hardly the place you’d expect to find a weapons cache, then. But my God, does it pack an explosive punch.

Entry-level Price£82,495Price as tested£84,245
EngineSupercharged V8, 5000ccTransmissionEight-speed auto
Power542hp @ 6,500rpmTorque680Nm @ 2,5-5,500rpm
0-604.6 secsTop speed186 mph (limited)
Economy22.2 mpgCO2297 g/km
Dimensions4966 x 1939 x 1511 (LWH)Kerb Weight1967 kg