Two amazing classic Jag experiences in just three months is something of a treat, especially for this fan of the old leaper.

The first came back in March when, having just watched Andy Wallace pulling some big speeds in an F-Type, Jaguar’s people asked if I wanted to jump in the passenger seat of a D-Type with him for a two-hour schlep across northern France. I was there like a shot. 'Shot' being a relative term, bearing in mind the contortion required to get myself seated and strapped in. Totally worth it, though.

Another opportunity with one of Jaguar’s Heritage cars yesterday, this time behind the wheel. A long-wheelbase, Series One XJ12, to be precise, finished in deep red, or Royal Claret, to give the colour its exact name. A name, perhaps surprisingly, free of hyperbole, because this particular XJ was once owned by Queen Elizabeth, the Queen Mother.

It was built and delivered to Her Majesty in 1973, and once its time with her was over the XJ12 was retired to the Heritage collection, alongside several other Jaguars and Daimlers once used or owned by the Queen Mother. Fitted with Jaguar’s iconic 5.3-litre V12 engine, it kicked out 269hp when new, and was once, Jaguar claimed, the fastest full four-seater in the world, with a top speed of over 140mph.

It bears the marks of its former owner, too, the leaper absent, and the flag that once stood in its place replaced by two rubber bungs covering the holes. And it’s not just special because of its royal patronage, either; this XJ also has the honour of being the first ever long-wheelbase example to be produced. Apparently Jag built it as a one-off for the Queen Mum, but then came over all “hang on a minute lads – I’ve got an idea...” – and before you knew it, the production LWB was born.

What’s it like, then? Well, you know what you’re thinking it’ll be like? Yeah. That. Open the door and you drop into a seat set low; the driving position rakish, almost coupe-like, and miles removed from today’s taller limousines. The dash, meanwhile, is that solid slab of pure timber, edged in padded leather, that no Jaguar of its day would be seen in public without. Crank the engine and it simply clicks into life. Into D and the big V12 swells beneath you, and you’re off, seamlessly. Acceleration is muted, but the XJ does a sterling job of gathering speed whenever required, the kickdown giving an extra bit of punch to spur the old girl up hills. Corners are dealt with sedately, meanwhile. The sense that the spindly Bakelite steering wheel is attached to the front wheels by means of little more than cotton wool and hope sees to that, but the result is that progress is never so fast that it becomes ungainly. And beneath it all, that lacquer-smooth V12 rumbles and purrs softly to itself, murmuring happily each time you prod the throttle.

It’s utterly graceful; composed and elegant, without being ostentatious, and still more refined than many a modern car. A vehicle that perfectly befits a royal, in other words. And while its provenance makes it unique, even without that it'd still be a very special thing in its own right. It’s easy to see now why the XJ caused such a stir when it first arrived. And sitting behind the wheel of this one, I don't doubt Her Majesty must have felt rather proud of what her nation could produce.

Alex