I replaced a couple of windows with more original looking sash windows. The sills were cut by Quince of Biggleswade for roughly £250 each from memory (2016 ish prices). Their website is all about granite worktops but I can find them if I search for stone masons in the local area and the same search might work near you. It's worth going new for sills - I ordered three and they all needed to be different sizes. When you install them they need to fall to the outside to drop rain.
For windows probably a smaller local woodworking shop will be the thing. I had 5 made at the same time and they came in maybe £1300 each.
If anything the brickwork people will be the most difficult to find. Any brickie can modify an opening and fit a sill. There aren't so many that will do it properly with lime mortar
with absolutely no cement in it. Worth phoning around for that one.
Here's mine for inspiration. Top window became a proper sash and got an arch it never had before to match some later windows on that facade. Bottom window closer to how it was originally (still with the original arch that has moved a bit). Both had wooden sills and now have sandstone sills to match the rest of the house.
What a difference! I feel inspired. I will take on board all the information/advice everyone has given. I've learnt a few things too. Thank you to everyone who has posted. I was extremely stressed but I'm a lot calmer now. What's that saying? Knowledge is power. I also know that if I need help again, it will be here for me