I'm waiting for a reply from the owner of the painting, the wife of the son of Hilda Moore, whom I've asked if she has any additional information about the painting. Unfortunately Hilda died when her son was only five, so, he probably barely remembered his mom, and certainly would not have known any stories about her friendship with Ernest. But possibly there are some notes or letters or something which might establish when the painting was made and given. Hilda and her sister Eva were some of the few friends at Ernest and Janette's wedding.
Edited to add: I receive a lovely email from Mrs. Fairman; turns out her husband worked on a TV program in which Ernest appeared, and they talked together about Hilda. Ernest remembered Hilda's son when he was a toddler.
No definite date for the painting can be established because Ernest knew Hilda for a long time.