Andy,at the moment there is not a proper sill brush on the market,some are using the big heavy high/low vehicle cleaning brush as the head is shaped so the bristles face forward and down,
i have approached my supplier about getting a purpose built sill brush,
What i and others mormally do is to clean all the windows then adjust the neck so the brush faces down then wipe over the sill,
and i do all windows up and down wfp,
Gaz