Sorry this happened. I had something similar happen to me, minus the “cooking” . On brand new recovered seats, too. I took the car to the body shop to get some corrections done on the strip/repaint they had done. The car sat there for a month, and when I picked it up, I had a meltdown to see my new leather seats with water stains. Apparently they had been working on the car with the top down, some water pooled and stained my seats. But the leather remained soft. I actually just got new leather seats again today. If your leather has become hard, I don’t think there is anything that can repair it. The best solution may be to look for a used cushion with similar patina? Is your car Barley?
A few points:
There have been variations in the leather used by Jaguar over the years. I’ll not claim any data points, but one aspect is the colouring method that went from vat dyed (the leather is fully coloured front and back) to surface dyed (very typically late 90’s seats would wear very poorly) to more current, where the colour stays firm. I’m not sure when Connolly moved from bart tanned to veg tanned, but I don’t believe it was too recently, as veg tanned leather is very much the child of the environmental focus on traditional tanneries and their horrific ways. So it’s not exactly apples all the way, yet we expect the same products to work.
When it comes to how our seats have been taken care of, there are massive variations there as well. Unless you have certain knowledge of how the DPO(s) have treated the leather, make no assumptions. The range of products and contents are vast, a number of them even include silicone, which while it gives a shiny surface also fills the pores with residue that will block any further endeavours to maintain the leather. If they’ve even used anything at all, as modern leather appears to manage just fine with no attention at all, and certainly within the lifespan most expect of modern cars.
And then there’s the matter of whether the leather has been redyed. There are various options there, with various advantages and drawbacks, but eash will take the resulting leather further from what it may have been.
At the end of the day, a seat that has seen plenty of use, possibly not too clever maintenance and maybe a layer of latex recolouring on top, isn’t going to be really receptive to some quick rejuvenation, regardless of whether the product is even appropriate or what we think it may be (ref. changing formulas of traditional products). So what can we do? One option is to really clean the surface. There are products made for this, again a varied range from saddle soap (the name hints at the type of leather it’s intended for) through to harsher chemicals to remove silicone. These will naturally also have an impact on the surface colour of the leather, so step carefully and YMMV.
Something to consider is treating the other available surface, the backside. Seat cushion covers are easily removed to test the principle. That untreated underside will absorb in an entirely different way than the gummed up topside (where if you are lucky it’s actually the stitch holes that are allowing the product to enter into the leather cell structure). For this kind of work I like to use neatsfoot oil (actual animal oil, so it’s at least similar to the stuff that was there when the cow lived in it) and with careful application I have had leather soften up. It pays to be a bit careful though as you don’t want to make the leather sodden with oil, it takes forever to stabilise again.
Take a moment to consider the life of a car interior: Extreme variations in temperature, regular applications of salty, sweaty backs, pummeled by posteriors, spillages and slippages. It’s no wonder they show some wear over time. To imagine that a quick dab of “hide food” will undo the damage is pretty hopeful! It’s akin to finding a pot of cream that promises to unwrinkle our worn out faces with just one gentle application. We know that’s too good to be true, right?
Thank you Nick for the thoughtful response. You’ve given me some ideas to consider . I’ve only done general maintenance on the seats with wipes, but I think I’ll go back to the basics with a good cleaner and conditioner and see how it goes from there. Steady as it goes as they say nothing too harsh.
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So how does that product differ from the one with the same name in the original poster’s pic, Gregmatic? The bottles look different, I admit … 
Note what Wilkinsonbladedude said above, which I think supports my comment (but not sure 
) 
Eh? Different patents, different results. I have always had success with Lexol, getting out dirt and stains, and protection. I really like the smell too.
But does it soften the leather, as the original poster was looking for?
Yes. Try it. Yes. Try it
Maybe it’s just different packaging or marketing but I’m will to give it whirl and test the results. I must admit I’m really annoyed that I allowed this to happen in the first place but there it is.
JagGrrrrrrrrrrr … I ran across another leather conditioner product (liquid) today at WM … It’s made by Jay Leno’s Garage … Looked very interesting, but then it better do what it claims for $13.00. 
I have never purchased any of his line of products, but I would think Mr. Leno would not put his name on something if it didn’t do what it claimed to … ?
I like to have several things at hand
I always come back to Lexol. All my cars absorb (not sit on, not evaporate, but ABSORB Lexol.)
I just bought some Pinnacle Black Label Hide Soft Cleaner and Conditioner that will come in Saturday.
I also keep Griots 3-1 Leather on hand
Lexol , of course
I am thinking of ordering Swissvax Leather Milk but it might just be over the top.
I guess I enjoy auto leather care products.
(making note: MarkJ = really into leather stuff) 