Complex sentence: The first Sovereign was a luxury version of the Jaguar 420 saloon, itself based on the Jaguar S-Type with a revised four-headlight nose reminiscent of the Jaguar Mark X and fitted with the 4.2 litre version of the straight-six XK engine.
Split into simpler sentences: The first Sovereign was a badge-engineered version of the Jaguar 420 saloon, which was itself based on the Jaguar S-Type. The 420/Sovereign differed from the S-Type in having a revised four-headlight nose reminiscent of the Jaguar Mark X and a 4.2 litre version of the straight-six XK engine.