Could be an idea in to looking into getting your sash windows draughtproofed as well. If you're a DIY enthusiast it is relatively easy to do with basic instructions or call in the specialists. It's something that English Heritage endorse and means you keep your original windows.
A joinery shop will probably cost about £130 for the sash lite and £70 to glaze, or cheaper if you can glaze yourself. Definelty replace as opposed to renovate as you're probably looking at £150 in labour alone.