About 10 years ago I had UPVC doors & windows installed with the front door being a Paladin composite unit. The glazing retaining trims have become severely yellowed on the outside - the inside ones are fine (see attached pictures). I cannot trace Paladin so it seems they may have gone under & I'm not getting much joy from the installers so can anyone advise if it's possible to return the trims strips to their original white colour? Ordinary household chemicals including bleach don't do anything