Mike I tried to get into upvc restoration a few years back, but to be honest I didn't find any product that did
the job to my satisfaction.
Some will clean ingrained dirt and help a little others will put a shine/coating which will make them look better
for a while like waxing a car, but then you run the risk of this of this coating running onto the glass and spotting if your wfp.
But even for the small gains its hard time consuming work and to be honest better left alone especially if they are really bad.
Im talking about white only for brown theres nothing that will restore the colour.