Hi Steve
Don't put any soap, detergent or your applicator / mop anywhere near them - they will smear like hell especially when the sun gets on them, the soap will stay in the edges of the lead, and in summer you'll be rubbing and rubbing like hell.
If they've not been done for ages:
1 ) Bone dry piece of old washed out towel or similar so its really hard or heavy duty paper towel or even old paper and scrub all the muck off DRY (it comes off easier and leaves less mess).
2 ) Go over with a damp sill cloth and get the muck off.
Then change cloth.
3 ) As Panorama says damp scrim - dry scrim - although I have been experimenting with damp microfibre - dry scrim - that seems to work but you can't beat a massive piece of damp scrim especially in warm weather it stays damp much longer.
If they are done regularly skip step 1)
Just my way of working and I have never had a complaint with leadeds
G.