Henshaw left a lot of material to the museum.
Skittles is right.
From the RAF Museum website:
For curatorial enquiries relating to the Aircraft & Exhibits Collections, Fine Art Collection, Medal & Uniform Collection, Film & Sound Collection and the Photographic Collection:
curatorial @ rafmuseum.org
020 8358 4873
Fax: 020 8200 1751
Re the suggestion of trying to contact Alex Henshaw Jr:
Unless it really is important, I'd advise against troubling him. He and his wife aren't in great health and I suspect that, if Alex Snr had a copy, he would have given it to the RAF Museum.