Rejuvenating a window & the final puzzle is how to mount the new window ledge.
The ledge is timber with a tenon which fits into a mortice slot in the timber window frame.
But when slotted in there is a 6 to 12mm gap underneath the ledge to the underlying block & brick work (which spans across the cavity & butts up to a dpc against the outer leaf.)
Q: How is this void best filled?
TIA