This old thread contains a lot of useful infomation, but also references and links which may have been updated or replaced by newer sources.
The UK devoted a lot of effort to finding a lightweight family of vehicles and eventually fielded the CVR(T) which has like the FV432 served long after the Cold War. That makes these early studies especially the wheeled alternatives and the Contentious light tank interesting.
Having received and read David Lister's The Dark Age of Tanks I found myself interested in project Prodigal/Contentious again. I also found two interesting contemporary photos online showing the gun on the demonstrator at maximum elevation/depression and traverse. Most of the available information on the vehicle for which the Comet chassis was the demonstrator appears to come from a single 1959 file at Kew, I have not read it myself but I suspect most of the useful content has been reproduced across multiple books and this website, which has some diagrams of the ribbed armour taken from that file. Poking around other books it seems this particular line of enquiry ran from 1956 to circa 1964. Almost every feature of the concept was novel, as if a 20-30 ton air-droppable SPG/tank with a 120mm main gun wasn't ambitious enough. It also would have required a bespoke engine to be designed for it as no existing British engine had the required HP and size format.
A couple of mock-up photos, David Lister states that at least three were made and these photos look to represent two of them. However, the mockup chassis seem to be identical, only the turret looks to be different. The damaged one seems to have been turned into a range target though I suspect this was after it was surplus to requirements and was not it's original purpose.
Thinking about the engine for Contentious, the most obvious candidate might be an uprated Rolls Royce K60 with a turbocharger, as this 1962 Commercial Motor article describes it was always intended to develop the engine to give in excess of 300hp. Such an engine was considered for the Swedish S-tank as described on the Swedish Tank archives website. A turbocharged variant giving 275hp was sold as a generator for mobile satellite ground stations, I assume Skynet. A K60 with turbochargers and intercoolers was run at 500hp experimentally but I don't know any details of this. The other option would be to take the Leyland L60 and chop off some cylinders, perhaps two or three to produce an L40 or L30 in the same fashion that Rolls Royce produced K40 and K50 engines with the same cylinders as the K60.
And another picture, this time of the demonstrator in standard posture.
Further thinking this through, and having read and re-read the relevant sections in both William Suttie's The Tank Factory and David Lister's The Dark Age of Tanks I am now very curious about the proposed autoloader. The options outlined by David Lister are clearly not for the Contentious 2 concept, they are meant for fixed installations as shown in the book. Options for autoloading the Contentious 2 gun with its 40 degrees of traverse would have been limited to bustle mounted systems, something like the AMX-13 but with the need for the oscillating turret removed by the use of the suspension for elevation. Given the compact nature of the design, fuel in the front of the hull, crew and turret in the middle and engine at the rear ammo storage in the hull would have been very limited and I can not imagine a mechanical means of getting it from the crew compartment to the turret rear where an autoloader would have to be. So, would a pair of rotary magazines, as on the original 1954 concept, but mounted inside the turret have been the most likely solution? Would this have been derived from whatever the loading system was that was considered for the Chieftain?
This design is particularly interesting, it looks like the contentious crew concept with Driver and Gunner in the turret but with the autoloader replaced with a person in a rear facing seat. The drawing, based on the scaling, looks to show a 105mm gun which may be a scaled up version of the 76mm L5A1 low velocity gun used on the Saladin - both calibers being mentioned in the text. The turret has 180 degrees of traverse versus 40 in contentious and it lacks the fuel tank-glacis configuration. In short, it looks like a cousin to the Contentious concept. The Swingfires are interesting.