As it says really. My 8 year old van seat isn’t as comfy as it used to be so would like to add some sort of seat pad. Halfords have some and I imagine there’s loads on Amazon/eBay but I’d like something that will be comfy and fairly long lasting. So your recommendations please.

Is a new seat base from the dealer or a scrappy not an option? That would be my first choice, or re-foam by an upholsterer.

Replacing the foam is fairly straightforward on most cars, the cover is held on by hooks and velcro, or just stretches over the ends of the frame. ~£70 and it’ll be like new.

The problem with any pad on top, is it’ll tend to sag into a U-shape and roll you into the middle, whereas a big block of foam with springs under it sags vertically an supports you. Like the difference between cheap and pocket sprung mattresses.

Caveat that opinion is a mix of reupholstering and re-foaming my old MG, and taking apart a Ford Fiestas’s driver’s seat for an unrelated issue (fixing the tilt mechanism). I’m assuming the van would be as easy to dismantle as a fiesta was, and the benefits as good as putting decent foam in the MG.

As said above, try manufacturer or eBay for a new seat base/ replace the foam.
I’m an upholsterer and in my experience most car foams are moulded so not easy to copy.
I got one for my transit for £40, took about an hr to replace it.
Zip ties are your friend, a lot of car seats are held together with hog rings, like a metal c.