I know the type of stain you refer to Ian, Try sugar soap on sun stains, where the sun has basically bleached the PVCu and it won't shift it.
A member on here several months back had tried to shift a stain on a front door that had been bleached by the sun, he used loads of products mentioned by members, including sugar soap, Nothing would shift the stain, He then tried the Cellulose Thinners and shifted the stain in seconds.
Cellulose Thinners is also very good for the little sucker marks left by creeping Ivy, if you try to shift them with sugar soap, just make sure you price well because you'll be there for a long time.
A little test for you Ian.
One door with a sun stain on it (often a deep yellowish stain) clean half with sugar soap and time yourself, Now clean the other half with Cellulose Thinners and time yourself, I won't say anything but let me know the results, not only for the time taken but for the results of the Job.