We have a product here in the states called "Fast Orange" hand cleaner, with orange oil and a small amount of finely ground pumice in it. There's probably a proprietary version available in the UK. I've used it for stubborn grime and discoloration on vinyl, and it works well. be gentle, and use a very soft scrub brush, and follow with soap and water. It will work particularly well on basketweave vinyl. Good Luck,
John Scribner