I buy my Cream Cleaner from a local upvc window supplier. Std Siroflex Cream Cleaner about £2 a bottle. Also use Dockta Power from Asda, Poundstretcher etc.
With Cream cleaner I wet the frame rub cream cleaner with Green/White non scratch pads, (from Asda) leave for 5 mins then wash off. Really stubborn stuff I get the Dockta Power out. Don't use Solvents, seen a frame a lad did recently and there were large brown patches all over it.
Also use the Magic Sponge (Dockta Power)
Wayne