There is no arguing with data like that ... they are the results I would have expected ...
It occured to me afterwards that I probably hadn't made it clear that the problems I had described only occured in a tiny percentage of plastic greenhouses (although large enough for part of a college lecture on greenhouse materials to be given over to the subject) sited where a confluence of factors come together. Remembering back to the discussion Richard and I had last winter (?), and his later photos, concerning the positioning of his greenhouse it seemed possible to me that he may be encountering such issues. These conditions are very rare- I have never encountered them or heard of them directly.
One solution that Richard may be able to adopt, and springs from Jenny's comment regarding Specific Heat Capacities, is the use of containers filled with water to absorb heat during the day and release it at night. A grower friend of mine had a lean too plastic greenhouse (the 'lean-too' wall was cement/breeze blocks which would also have absorbed heat) in which his benches were stood on 220L drums filled with water. Even though there was no other heating involved he was able to produce a
merchantable crop of basil right into December even though night-time temperatures are commonly as low as -10C. Being so far south of the U.K. helps of course but I would imagine that water filled drums in the U.K., even with Richards sheltered location, could absorb enough latent heat to keep tender plants from freezing during the night.