premise: The birthday party was of a kind which Cecilia would have wondered at, had she been invited to it, for although a few of Bienvida's classmates were present in its early stages, by seven it had become a celebration for the grown-ups. Bienvida and Jasper were of course still there. No one noticed that Bienvida her grandmother's granddaughter more than her mother's child had at some stage gone back to the Headmaster's Flat and changed into dungarees to keep her Oxfam dress of nylon organza from getting dirty.
hypothesis: Bienvida had at some stage gone back to the Headmaster's Flat