Hi Mac
I have always found that even on really badly stained PVC, the pure water will lift 95% of the staining out.
Any really bad patches I find, I will soak them down, leave them for a couple of mins, then give them an initial wash down and another good soaking. This seems to work well.
If you are going to use a cream leaner, rather use a UPVC one or a very soft (not abrasive) cream cleaner, otherwise you will end up scratching the surface. Scratches may not show straight away, however, within a few weeks of weathering and dirt, they will begin to show and your future clean of the UPVC will be 100 times harder.
Hope the above helps
Many Thanks
Andrew