You can find out more about Stewart in the following archives.
Stewart’s commerical work, cartoons, small works, ephemera and almost ten years of correspondence between him and Charlotte Cooper have been archived at the Bishopsgate Institute. Anybody can visit the archive and have a look at his work for free.
Before Stonewall: A Lesbian, Gay, Bisexual and Transgendered Oral History is the project in which I interviewed Stewart. The British Library holds the whole archive, which features over 100 video interviews with older LGBT people. The original interview tapes are lodged at the Screen Archive South East. You can go and watch any of the interviews by contacting the librarians and archivists directly.
The Imperial War Museum holds Stewart’s account of his experiences fighting during World War II (June 1998). Stewart also gave me a copy of this text, which you can right-click to download and read (.pdf, 33mb).