There are various storm windows with good tight seals that can be placed on the outside of your current windows. This will keep the inside pane warmer and stop the condensation. However the leakage from your old windows can cause frost and condensation on the new storm window. The better more, expensive method, if you really want to keep your antique windows, is to sandwich them with a storm window inside and out.
Thank you - that's really helpful. I would love to try your method of sandwiching but it may be prohibitively expensive as there are a lot of large windows. May try it out on one to begin with and see if that cures it. Do you think this http://www.stormwindows.co.uk/pdfs/spec_horizontal_slider.pdf product will stop the condensation or will there still be leackage? I am loath to spend a lot of money on secondary glazing if it doesn't cure the problem permanently.