ok ok
you will require
sanding block from b&Q medium - coarse grain, dont get a block of wood and put sand paper round it! the block i use is green and only a few quid.
cream cleaner from tesco
cloths
wipe the sill clean first, then aply the cream cleaner, work with about a foot or two at a time, use the sanding block back and forth, press hard and rub away with pleanty of elbow grease. rinse with a hose then wipe dry, trust me it works fine, ive done this at least 10 times over the last 2 years, works every time.