For Cleaning PVCu frames and sills, but do this at your own risk & always try a test area first.
I've been using "Standard Cellulose Thinners" for years without problems. You can buy it from any car paint suppliers (specialist dealers, rather than Halfords etc.) It's about 7 quid for 5 Litres. Absolutely amazing stuff. I've never noticed any ill-effects (try it first though) but use white rag to wipe the frames with it. Coloured rags may bleed colour. Keep it away from rubber beading (or you'll end up with black streaks on the white frames). Don't worry though - any black streaks or colour bleed from rag will simply wipe off with the thinners. Wear rubber gloves, preferably ones rated as chemical resistant. Marigolds tend to split after a while. PLEASE NOTE THAT THIS ADVICE IS FOR WHITE FRAMES ONLY!!!!! I'd be wary about using it on brown frames etc. Use in a well ventilated area (I.e. Outside) and DON'T SMOKE!!! If you spill the thinners on any finished surface (best coffee table etc. It'll spoil it before you get the chance to wipe it off quick).
P.S The thinners makes brilliant paint brush cleaner too, but get it on the handles of your best quality red wooden handled Harris brushes and it'll strip the finish off before your very eyes.
When you apply it to any PVCu, apply to your cloth, wipe it on, the wipe off immediately.
When the Job is complete, apply some car polish (any car polish will do) but I use MER polish available from Halfords to give a good lasting shine and a protective coat.
Be careful not to use anything that contains an abrasive, no matter how mild. It will scratch the surface of the PVCu
Any surface scratching, no matter how fine, will attract and retain dirt and be very difficult to clean properly in the future.
CIF, I believe, does contain a mild abrasive so should not, IMO, be used.
If you have any small scratches, stains, etc on the PVCu these can be removed with 2000 grade wet 'n' Dry.
Wet the paper in a bucket of water and begin to rub the scratch and only in one direction, this direction MUST be along the length of the sill and not across its width, occasionally wipe the sill with a clean cloth to check your progress and continue until all scratches have been removed, Once your happy with the results, dry the PVCu with a clean dry cloth and apply car polish to shine and protect.
I have restored a lot of PVCu using the above methods and results have always been first class, so I know they work.