Jaguar took a huge leap forward when in 1955 it launched a compact 2.4 litre unitary construction saloon, retrospectively called the ‘Mk 1’. But when its replacement was due, it’s said that Jaguar founder Sir William Lyons wanted more than just a worked over saloon; he thought that the 1960s would demand a car of greater refinement. But time and costs precluded an entirely new model and so, by 1959, a facelift was called for and the saloon lost its thick pillars and doorframes to evolve into the much airier Mk 2.

At that time, Jaguar had been experimenting with an independent rear suspension (IRS) system destined for the now legendary E-type, followed by the huge Mk X saloon, both in 1961. But although tried on the ‘Mk 1’ the IRS did not sit comfortably in the Mk 2 where space restriction hampered a successful installation. So at last a new model could be built, sitting somewhere between the Mk 2 and Mk X. Project Omaho was given the go ahead and a car would be built around the existing Mk 2 platform – the S-type was born!

Essentially the main cabin area was retained from the Mk 2, however the rear end substantially re-worked; not just to accommodate the suspension ‘cradle’ but also to improve refinement and remove as much structural-borne noise as possible. The seats were mounted slightly lower in the floor and redesigned to offer greater comfort while becoming slimmer to gain internal space. The little picnic tables of the Mk 2 were deleted. With the strategic use of four big rubber mounting bushes securing the main assembly and a pair of bushed radius arms to control movement, the whole system was isolated from the main structure. A certain amount of double-skinning also helped to detach the occupants from outside forces. Rear brakes went inboard to reduce unsprung weight.

Obviously this involved a certain amount of body redesign, evolving into an elegant tail section, now with a separate fuel tank in each wing. The front end was largely left alone although the nose was given cute hooded nacelles over the lights, to strike a balance with the rear. The well-proven front suspension unit was a virtual carry-over from the Mk 2 although brakes became slightly larger and the power steering system improved. In a final balancing exercise, the roof line was flattened towards the rear, complementing the front and back revisions. This entailed the use of slightly different door-frames too. Unlike previous Jaguar saloons the bumpers were elegant slimline items subtly blending in with the revised body shape.

In 1963 the S-type was announced with two engine sizes - the 3.4 and 3.8-litre both with the B-type cylinder head and twin 1.75-in SU carburettors. Transmission was both manual and automatic. Wire wheels became an option. Fully upholstered in leather, the interior was very luxurious and, where the centre dash of the Mk 2 was black vinyl, the S-type used veneer.

On the road it becomes obvious that the ride was considerably superior to previous Jaguar models. Performance was slightly compromised by the extra weight but, in day to day use, it was barely noticeable. In fact, so good is the rear suspension that an S-type will always out-fumble a Mk 2 around those twiddly bits, making it a quicker A to B car. Straight line stability is excellent too while the dual exhaust system keeps mechanical intrusion at bay. Little in the way of production changes occurred during its life, the most significant being the replacement of the Moss gearbox by Jaguar’s own all-synchromesh type in 1964. Cost cutting came into play in 1967 when the fog-lamps were deleted and Ambla replaced leather for the seats.

With the arrival of the 4.2-litre engine in 1964 Jaguar didn’t just replace the 3.4/3.8-litre engines, it created an entirely new model – the 420. But it had to wait until 1966. Substantially based on the S-type the engine bay was slightly widened with greater bonnet access to accommodate the 4.2-litre engine, fitted as standard with Jaguar’s straight port cylinder head and twin 2in SU carburettors. While the rear remained pure S-type, the front was re-shaped to follow Mk X lines. Superb three-pot calipers helped the braking effort while the excellent Varamatic power assisted steering allowed a progression of the steering ratio giving more power as the wheel was turned. The interior remained largely untouched, although padded tops were employed for the dash and doors with a neat clock in the centre of the dash top.

In a slightly bizarre marketing ploy, Jaguar actually filled the showrooms with 3.4 and 3.8 S-type alongside the 420, separated by around £500. It had in fact created its own rival as little from other manufacturers could match the Jaguar. Sales of the Mk 2 derivatives were falling and with the 420 on the scene, few wanted the S-type. In fact the 420 became Jaguar’s best seller ahead of every other model including the E-type.

No Daimler variant was ever offered in S-type mode but Jaguar deemed it appropriate to badge a 420 as Daimler Sovereign. Jaguar had played the Daimler game before when attaching the waterfall grille to a Mk2. But at least that car had a Daimler sourced engine, the Sovereign was pure Jaguar, albeit with every extra box ticked. The ploy worked as where the 420 may not have appealed to the traditionalist, the Sovereign did and the name was even carried over to the XJ saloon range.

It is a curious fact that, although the Mk 2 (and latterly the ‘Mk 1’) have soared in value, the S-type fraternity has trailed behind. And yet it is widely believed to be a substantially better car, with the 420 the ultimate development of the so-called compact range. As time progresses, though, many are catching on to the charms of these splendid classics making them a very easy Jaguar to live with. The 3.8 S-type survived until June 1968, while the 3.4 production ceased in the August of the same year. The 420 quietly slipped away in September, just as the sensational XJ6 was announced.