There are all sorts of favourites if you check through earlier posts - ranging from traffic film remover (which I haven't used) cif, own brand non abrasives eg eurocell b and q
There's also the spirit based stuff which I find is only good for marks on new windows ( I used it when I used to fit them) -
Then there's the waterfed pole and pressure washer
Wouldn't mind trying pressure washer at some point but having used the pole and several cleaners I cannot see how anything will get them back to original - abrasives pads/cleaners just invite more dirt next time as it sounds like you already know.
Honestly think it is abut educating the customer - I always tell them that upvc is like tupperware - you cant rough scrub it or you ruin it and unless it has been kept clean from the off, you won't get it perfect - so far they all accept this