Westleys Bleach white, that all you need....................Jim

It works really well, but doubt he has Dollar Stores in Denmark.

Starbrite Extreme Clean works very well. Kind of pricey, but designed to be used on boats and vinyl.
Boat Bling Vinyl Sauce works great too.

Here it is...

I started this post, and have after seeing that advice above, I have now pretty nice white seats in general. But I still have one specific problem. My boat unfortunately is parked outside in damp Denmark winters, and when taking the cover off it after a month or two, the white seat areas where the black cover rests on are dirty, black patches. I can normally clean 95% away, but always plenty of very small black dots stay, can't get them off. A kind of fungus? Don't know. Is there any good chemistry that can make the seats all white again?

so which one is better ? starbrite or awesome

https://www.fastenal.com/products/details/0601166

I use this, it is the best that i have found besides Whestly's bleach white and a magic eraser which are pretty hard on the vinyl

So have you had those small black spots, and been able to remove them? With the above?

yes, let it soak for a few seconds and scrub with a soft brush.

Not sure how to tell you to get the rest of the black spots out, but I can tell you it's probably mildew coming from the underside of the vinyl. If you ever have them recovered be sure to wrap the foam in plastic to prevent that from happening again.