Why not contact a company like this to see if they can help?
Window And Door Components | Joinery sections and profiles (wooduchoose.com)
Window Frames & Cills (terrystimber.co.uk)
Google's your friend in situations like these. The lesser of two evils might be to order a completely new window i.e. including frame.
Google isn't your friend when you didn't know the name of the bit you needed. But now I know I can probably work it out.
I found the original manufacturer, who changed names (which is always suggesting they are trying to escape their past) and I have contacted them. The new buildings they make are almost certainly the same design and they are not that far away - so we'll see if they come back. I doubt they will, because thats how things are these days.