I think the best is a mixture of washing up liquid and bleach. Leave it for a few hours or over night and then blast it with a power washer. If the finish then looks clean but dull, wipe with some dash board cleaner or a thin coat of wd40.
i bought product in aldi a few months ago called PVC restorer.
i've only tired it on one window but it seems fairly good. i think it was around €8.
you could also try a cream cleaner like Astonish (has been mentioned in relation to cleaning hobs) or maybe the cif cream cleaner that has added bleach.