Googling produces lots of diagrams of sash windows and sills, but
nothing that I could find that answers my question below. Please can
someone advise me?

Normally, does the stone window sill in a typical 1880s victorian
terraced house (with sash windows) extend right across the whole depth
of the (8") wall?

I need to get a replacement sill from an architectural salvage yard
but I don't want to expose the old one (to measure the depth) until I
am ready to actually do the replacement so I'd like to estimate the
depth I need.

and is it sill or cill?

many thanks,

Robert