This is for John and Angus. There is a distinction to be made between polishes and waxes. The former do contain abrasives, and some such as Mer leave an annoying white residue, which tends to cling due to static build up through the buffing action on the plastic window. If the window is uncratched, it would be better to use a wax.
As I mentioned in my previous post, I have found Meguires waxes to be very effective in removing even the very fine swirl marks that polishes such as Mer leave, and which are particularly noticeable on dark colours. There is even a 3 stage/product process to achieve the best finish you could wish for - but its a bit OTT for caravan windows...