Hassall died intestate. His working papers were taken to the Royal Society of Literature by his assistant Gillian Paterson, who arranged and (presumably) listed them. They were bought by CUL from Bloomsbury Book Auctions in 1992. They were housed in paper box files 1 25 and cartons A D, as listed. They have now been re boxed, in 28 boxes and one packet, which has involved some reorganisation of the material and hence some rearrangement and re numbering of the original list.

The collection largely comprises letters addressed to Sassoon although there are few other recipients such as Glen Byam Shaw and Georgiana Theresa Thornycroft, Siegfried Sassoon's mother. There are also some postcards; draft and unpublished copies of poems; bills; and other related papers.