Back at home I read up on the history and found that the re-development of the Serpentine Gallery was completed in February 1998. ChatGTP put it as follows:-
Concerns about UV damage, thermal instability, and inconsistent lux levels pushed many galleries toward fully controlled artificial environments, even in buildings never designed for that approach. The 1998 refurbishment, led by John Miller + Partners, followed this logic closely - prioritising conservation standards and curatorial flexibility over experiential or perceptual quality.