This temporary solution has been discussed before, but worthy of a refreshment.
I managed to find a UK supplier of the pins
who sent me 30 pins for £3.49.
Now lets see if I’m smart enough to add a picture…
Regards
Steve
And whilst I’m on a roll, UK alloy Wheel folks can buy this excellent product from Wilkinson’s for just £5. Don’t go for the smaller and cheaper type under another name, nowhere near as good!
Caution - Never apply this stuff to hot wheels, it will damage the finish. Cool them down with a hose first and let them dry off a bit before applying the stuff.
Well done Steve I’ve been getting them from China, it takes a month or more but they are £1.99 for 50 off.
Best thing i have found is a product made specifically for such headliners that strongly resembles those twist pins, called “Saggy Stoppers”. You used to be able to get a package of them at Pep Boys or Auto Zone, but I think now you have to get them on Ebay. SSers have the flat, clear heads on them and the “corkscrew” pin that twists into the backing board to keep them in place. 
btw, don’t get me started on Jaguar’s apparent inability to make a headliner fabric that will stay firmly in place for more than a few years. In the older XJ40s the problem was that the backing board - which was made out of a substance similar to styrofoam - actually disintegrated into small pieces/“dust”, requiring that it be replaced. The more recent models’ problem seems to be just the fabric letting loose from the backing board. I used to think that the headliner defect was maybe a problem resulting from our extreme Texas summer heat, but since then I have noted many of our friends across the Pond experience the same sad thing, and I would not think it would get that hot in the summer in the U.K… 
(next let’s talk about those &^)*&&)( sun visor clips that come apart due to age/heat 
)
You can also get them at any fabric store or places like Walmart or Target. However, it does require wandering into the hostile territory of the women’s departments. I’m much more at home in the auto and sporting goods sections.
We wouldn’t want you to feel on your own Paul, the most rare thing here would be to find a headliner that doesn’t sag. You’re amongst fellow sufferers across most of the JLR product range actually, it happens on Range Rovers etc, too, same cause, same old same old…
The days when a Car manufacturer cared how long something lasts after the 3 years or whatever warranty has expired are long gone, if they were ever there of course.
Happy New Year!
Steve
I’ve used these in my last two XJ’s. They work well. I was able to find a spray paint that matched the headliner color and painted the heads of the pins. They aren’t as noticeable.
I went by Wal-Mart last week to see if they had something similar. They do have those style of pins, but I’m afraid they’re not quite as “hardy” as the ones you have there. I was only able to get one to make a good purchase and go into the headliner backing board smoothly for about every 4 or 5 that I tried and ended up getting bent. 
Used a staple gun with 1/4 legs shot in at an angle on an old Buick LeSabre, the liner was really coming down. Looked rolled and pleated, no one ever noticed. 8 years later I gave it, still up where it belongs, to my co-ed granddaughter. I think I’ll go with the twisters on the Jag.