In the novel By the Pricking of My Thumbs, Uncle William is mentioned. He was an uncle of Tommy's, and had owned a desk, which was passed on to Aunt Ada, and later to Tommy.
According to Tommy, Uncle William was another man "who turned crusty in his old age besides being stone deaf and having a very bad temper".
In The Secret Adversary, Tommy has an uncle named Sir William Beresford. However, it is not known if this is the same person as the Uncle William who owned the desk.