Steve
I've cleaned similar cills and lintols, (a few years ago!) and I used a putty knife or similar to remove snots. Always keeping the cills very wet from the start! For stains, I sometimes used a carborundom(?) stone, and/or diluted brick acid, if you keep the cill wet and wash off the diluted acid as you go you should be okay. I also used a small wire brush(carefully) such as those used for suede shoes, hope this is of some help.