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Thanks to everyone who answered.
I live in a town in North Northumberland where there are many listed buildings and I've seen double glazing in some buildings that, you couldn't distinguish from the original. I would imagine it is quite expensive to install, though.
Secondary glazing could be the way to go but, don't get me wrong, life isn't unbearable without going down that path.
As SJ said, maybe I'm mad contemplating putting it in to somebody else's, property. Even though I would quite happily see out my days here.
Thanks again.