Brian Gaff (Sofa 2) formulated the question :
This is called painting oneself into a comer. He should have asked the
insulation question first. I'd have thought that rather obvious in your
description of the build. Seems an odd property to buy, unless it was really
dirt cheap.

and they were much younger and less bothered by the cold.

Of course a lot of people have lived in such houses for years and its only
as fuel costs increase the folly of the design becomes apparent.

He appreciates that had he done something about it long ago, it would
have easily paid for itself over the years in comfort and fuel saving.