I use a solvent cleaner,available from UPVC window suppliers,cost around a fiver a bottle,you need to wear a mask though because of the fumes given off,eats right into the build up of grime,but it dont really last that long,it does a couple of frames depending on the size,and you have to work pretty quick with it.Regards Brian.
Virosol.... It's the future... Keep yer garlic bread and get a 5 litre tub of this stuff...15 to 1 and go for gold...