I love that idea. The original windows are much bigger and more intricate. I would contact an architect in the first instance. You could also see if there is a heritage window company in or close to your area and see if they could send a surveyor to have a look. I would speak to a few builders too as you will definitely need one to replace the window. He will need to reinstate the tiled roof above. This is going to be costly, no question about it. Be prepared for a lot of disruption if you decide to go ahead.

You need a joiner. Contact a joinery company. They will deal with all of it for you.

Thanks everyone for their insights. I appreciate it will be costly. Do you have a sense of how much it could be, e.g. are we talking more than 10k?

If the costs were too prohibitive, is there anything I could do with my current window opening to make it more sympathetic to the original design (e.g. three wooden sashes? I'm struggling to find examples online)?

If you’re in the south east, The Sash Window Workshop is worth a look. They’ve done several jobs for us as we gradually return our windows to wooden sashes and are very good. Can recommend.

The Sash Window Workshop are fantastic. I used them in Winchester on an Edwardian house sash window replacement and everyone commented how good the replacements were. They were very professional too.

I love the idea to return the original window, however if that idea doesn’t come to fruition then taking out some brick rows from below and improving the height and style of window would go a long way to improving your frontage.

I like it as it is. Reducing the height of the window will make the room less private and reduce options for furniture. Don't be too precious about originality. I doubt the render is original, the doors and solar aren't and it's unlikely that any rebuilt copy will be an exact match to your neighbours.

Thanks all. One thing I've considered is replacing the existing uPVC window with something like the below, which would hopefully be cheaper than building a bay...

Bespoke wooden joinery to follow the curve of the brickwork? Timeless class.