The inserts turned out to be impractical to fit and the film he showed us was also VERY expensive.
As the film was originally designed to keep heat OUT of the conservatory, the manufaturers were understandably reluctant to guess at how much heat the fil would allow the conservatory to RETAIN.
I think the answer for me, at this stage, woud be to curtain off the conservatory from the rest of the house and keep the French windows shut overnight, not opening them until the temperature out there more or less matches that inside the rest of the house.