All that Windolene and Mr Muscle stuff(didn't know they made a window cleaning product) is rubbish. like you've experienced Posty,all it does is rub the muck into a paste and it stays on the window. You need a decent washing up liquid and some warm water. Buy one of those cheapo window cleaning kits with the squeegee and a blade. You need the blade more than the squeegee as that's the thing that takes the muck off the window,instead of leaving it on when you rub it into a paste with stuff like Windolene.
1)Dip squeegee(even a car washing sponge will do)into warm soapy water(about a tablespoon of detergent per half bucket)
2)Go up,down and across with squeegee or sponge(repeat a few times if hard to shirt stuff like bird sh!t)
3)Leave about 20 seconds for it to dissolve or soften the dirt(though if it's a warm dry day and the sun's on the window don't let it dry on)
4)Take the now dirty water off with the blade,though don't blade it twice as it'll leave a mark.
5)Finish with wiping the window gently with a piece of scrim(rough very absorbent cloth)
6)Stand back and admire your work!