Repairing an interior wooden sash window sill / stool

The stool is a separate timber moulding that fits to the cill by locating in a groove. This of broken can either be patched up or ideally a new one made to match the existing and replaced. Not a massive job to do but matching the existing moulding could take a little time

the fact that there sash windows could be as early as 1666 Yorkshire sash there commonly known as they couldn't open out over for the thatch and the great fire of London they became to be commonly used as the houses were so close they would be hitting each others house but you'd be best checking to see if it's protected or listed building but if it's of little significance {except to the owner of course} then generally they will be a simple curve or half round which would quite easily be reproduced and id only repair not replace unless its a necessity i even been known to go replace a fence after the posts and bottom of the fence was rotten 95% of fencers would renew the lot I cut 6" off a 6ft fence and replaced the posts and rails so the cills and widows won't generally be oak and are generally painted so it could be patched a bit of filler sanded and painted

Pull the whole length off . It’s probably fixed with 3 large nails so locating those first then drawing them out would make it easier and less damaging to the cill . Then you can replace with a bull nose moulding in one complete piece .