I hope you don't come unstuck :

If it's an old window, the lead will be very thick, so you can't just "wet & dry" scrim it - the lead will grab your scrim and stop you getting at the glass. The individual panes are small, so you will only be touching the centre of each pane, leaving a border of uncleaned glass each side of the lead which you can only get at by using one finger and treating each little bit of glass like a separate window, i.e. polish all round the edge of every single bit of glass.
It might pay you to lay out a waterproof sheet, use WFP and a "wet & dry" hoover and suck up the water every couple of minutes.
Personally I'd avoid it like the plague

Cheers,
Ian