Inherited it in my present house as all the old windows were leaded and falling apart. Dont know who did it though. Have you considered an opening or sliding UPVC or aluminium window/"door" inside if the window board is wide enough.Would have thought a box room upvc window would be no more than £350
Ebay has new secondary glazing options
you meen like a second double glazed window after the first one?
i did kind of think of that but thought i'd be laughed at lol.