Here's a guy taking an S type from the Jaguar Heritage Trust for a test drive;

 

very nice video. The hump inside the boot floor is to accomodate the rear inboard brakes, not the gas tank. The two gas tanks are each behind each rear wing.

If the doors are well balanced, you don't even have to push them shut, they close by themselves.

As far as the DG250 transmission, I find it to be primitive but quite effective. Third gear is almost like a Fourth gear, for highway speed, so you could say the forward gears are First, Second, and Fourth. Maybe that is the reason the "Second Gear Hold" switch was incorporated in the car, because Third Gear seems to be too "low" for city driving.

 

I have been watching some of his videos. He has one where he buys a Daimler xj40 and another doing a tour of the Jaguar Heritage collection (albeit a fast one).

 

Yes ~ the boot hump accommodated the entire IRS bridge piece. As you say the twin gas tanks are in the wings like the MkX/420G. He's a bit optimistic with the 2 carb 3.8 engine power. It was 220bhp (SAE) ~ only 10 more than the 3.4 although that did not tell the whole story with the greater torque of the 3.8. I don't quite buy the poor sales pop up of the S Type & 420 as being the reason for maintaining the Mk2. Rather it allowed Lyons to practice his niche pricing over a more broad range of models & kept competition like Rover out. Mk2 sales fell precipitously from 62/63 onward from the previous sales highs. But at least they enjoyed sales they might not have had as the S Type & 420 bridged the way to the XJ6.

I think the S Type styling has stood the test of time over Cyril Crouch's initial reservations & is reflected in their value over the 420 today. Radio was not standard. It was an optional extra.

 

Glyn, I think what most people first "see" and dislike in the S type are the rear wing wheel openings.
Compared to the MK-1 and MK-2 they are not as attractive, but they are the pattern that started with the E type, and the MK-X, then the S type and 420, then the 420-G, and then the Series 1, 2, and 3 XJ. The rear wing full wheel opening pattern of the MK-1 and MK-2 was not replicated in any future Jaguar.

In the US some S type owners have modified the rear wings to copy the MK-2 openings but it still does not work because of the longer length of the S type's rear third of the body.
To each his own, I could have had a MK-2 but decided on the S type because of the rear IRS and twin tanks. It simply rides better.
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My decision was also influenced by my XJ-6 which has been a wonderful ride for the last 30 years.