Use Auto Smart Ali (see there website) it is actually for alloy wheels but will clean UPVC better than anything anyone else will recommend here. There are some strict rules to follow with it as it is corrosive and posions - use gloves (you don't want brittle bones!) and eye protections, and make sure you have a hose pipe as if you leave it on the glass more than 5 mins at a time it will etch the glass, can dilute according to stubborness of stains and you should be able to get a 25l tub for about £100. I used 25l in 18 months cleaning an average of 3 conservatories a month with it. Best stuff by far and the UPVC GLEAMS!!!!!
Avoid aluminium frames with it like the plague as it CAN turn the frames blue!!!!
Good Luck with it