These are typically very small 'nicks' often where a seat squab or hatch comes up against a hard surface (or occasional SkySky foil

A number of years back I had a product called 'Vinyl Weld' similar to a tube of bostik, you simply made sure split was clean (acetone) put on a thin seam of the glue and it melted the 2 edges invisibly back together.
I can't find that on sale any-more ...... you can get the packs with 10 or so colours in them ... but they are nowhere near as good.
Anybody aware of a good product?
Somebody suggested uPVC solvent .. nor sure if that would work.