I’m currently experimenting with UPVC cleaning on my own house.
I’ve tried Thompsons UPVC cleaner does a great job but find it too time consuming to carry out the work as a service. Also tried TFR mixed 1/20 and scrim. This was a lot quicker. Will try WFP next week when I get my system.

I now have half my windows white the other half grey.

A bit of info I picked up at the Cleaning show from a chemist on one of the cleaning products stands was, if the dirt is so ingrained it is not possible to bring back to white.
ie: If you took a cross layer of UPVC (snapped in half) you could see the dirt penetrated to say 50% of the thickness of the layer, the ingrained dirt would be impossible to remove. Don’t know how true this is, anyone know different?
Wayne