I've finally got round to looking at one of my broken sash windows.
Basically I've had to spend a good 2hrs chipping the ancient gloss paint off all the working parts, just so I can see which bits do what. Anyway, following that I've managed to remove the beading from one side of the window, unfortunately in three pieces. I've also got half of the central (?) bead off, also broken. This allowed me to open the hatch and extract the two sash weights, both with broken cords. Now just popping out to get some sash cord and some other bits 'n' pieces.
Hopefully SWMBO won't mind the mountain of gloss paint chippings all over the place ;-))