After searching through all my Alan Bennett stuff, I have found the story of The Lady in the Van. It's in a collection of stories entitled, 'Untold Stories' published in 2005.
Thank goodness for that. It was really irritating me and I have posted this in case anyone else was sure they'd read it some time ago.
I have that book and have read that story many times. It's hard to believe it's a true story isn't it Tilly? ... but it is. AB put up with a lot from that awful woman.