Hugely common issue.
Loads over on d3/d4 forum and my friends XF just went on the near side…. They did that side and 10 days later the drivers side went! Both small hairline cracks in the plastic resulting in limp mode.

Both sorted now, although huge problems getting the rearmost injector out (took 6 hours alone)!

Has the plastic split ( material failure) or is it a seam that joins 2 parts (adhesive)? 2012 TDV8 Vogue (L322)

on mine it was a crack along the side of the manifold.

"Would anyone be interested in joining me in am action to reclaim costs?."
As someone who also had this issue (and bill) but on a 3.6, I wish you well, however I suspect this will be a long, frustrating and fruitless effort.
Playing devil's advocate, how long should a manufacturer be responsible for a car or component 10, 20, 30 years? There's an argument that if it has lasted for 11 plus years then it is fit for purpose.
JLR (and most other makers) have a long history of "known" faults, think about corroding panels & brake pipes, turbo drain mod, suspension airbags leaking to name a few.
I dislike paying for these repairs as much as anyone however I take the view that this is the risk I take when buying a car for a relative pittance compared to it's new price.
I expect any manufacturer will take the view that if you want them to pick up the bill for a repair then you need to by a new car with a warranty which isn't an unreasonable stance, if they accepted responsibility and costs for repairs on decades old cars then they would soon be bankrupt.
My own cynical view is that JLR wont pay out because
a, They aren't remotely interested in us who buy the older models as they make nothing from us
b, There are more than enough customers able & willing to pay the cost of a new one - that's where the profit is
c, We'll all moan about it then still come back and buy another one, the odd few who move on to something else makes no difference no matter how loudly we shout that "I'll never buy another one" - I've done that 4 times and still bought another

I agree that such an attempt would be futile. Hopefully the revised manifolds have been strengthened adequately although the V6 manifolds have been revised around 10 times (I'm not bothered to chase through all the part numbers again) but at least I've seen substantial improvements in the V6 manifolds such as bracing and extra thickness trying to overcome the cracking.

Class actions are rarely successful. You also need to be acutely aware that if unsuccessful, you can be subject to a counter-claim to recover their legal costs.

In terms of cost, you're talking hundreds of thousands of pounds in legal fees. I doubt members would be happy stumping up that much! Unfortunately, it's one of those things that does wear out over the years.

It may not seem just but unfortunately, that's the way it is. 2015 Range Rover Autobiography 4.4 SDV8