I take it that by "cottage pane" you mean the panes are fitted in lead cames, as per stained glass.

They can be converted to a more conventional glazing and it might be worth considering using 'slimlite' units which would double glaze them. The units aren't cheap (especially as they should be toughened in that location), but they do allow double-glazing into a single-glazed rebate and would be a one-off cost with long-term benefit.