The secondary glazing allowed both sashes to open and could be fitted with working shutters in place.
A professional and hardworking team, and a pleasant experience. I have large Georgian sash windows in my first-floor Georgian terraced flat) and I would also echo earlier comments about the secondary glazing being unobtrusive. It was also the only product that I could find for large windows that allowed both sashes to open and could be fitted with working shutters in place. Not cheap, but I’m looking forward to there being considerably less heat-loss through the large area of thin crown glass as we get into the winter months.