By coincidence I have just finished reading Diana Barnato Walker's "Spreading My Wings", and she tells a story somewhat like the one you are looking for, although it doesn't involve Beaufighters:
"Lois Butler, who had tried to dissuade me from even joining ATA, was stationed at No. 15 Ferry Pool at Hamble for a short time. She was trim, strong-featured, with sleek blonde hair. She had beautiful hands, was a vision of sex appeal, and an extremely competent pilot.
On 29 December, in the winter of 1944, Margot Gore tannoyed for a few of us, including Lois, telling us that a certain RAF squadron along the coast had suffered losses in aircraft so we were to take a batch of new Mosquito XXXs (mine was NT307) to them from various [Maintenance Units] around the country. These new aeroplanes, which were of a later Mark, would have slightly different flying characteristics to those which they had recently lost, so the RAF crews would no doubt welcome seeing girls bring them in without any problems.
[...]
Every one of us except Lois arrived safely, being certain to make careful - and beautiful - landings. [...] Whilst we were awaiting Lois' arrival, we joked and flirted with the young pilots, who asked us who was coming in next. Lois had married at 16, and her daughter, also 16, had herself had a baby. So one of us chirped up, 'The flying grandmother!'.
[...]
Lois did a dainty landing, taxied in and then emerged from the underhatch looking her usual immaculate self. She apologized for keeping everyone waiting. The RAF pilots, who had obviously expected some grey-haired, doddering old thing, were entranced at the apparition of femininity that had at last turned up."
As Treadigraph mentions, the Beaufighter does appear in the list of aircraft flown by Diana Barnato Walker at the back of the book, but I'm not sure it is mentioned in the text. I'll have a quick flick through tonight and see if there are any further clues to the mystery!