I discovered quite a lot through contacting Sue Light of the WW1 nursing website Scarlet Finders. Sue suggested I contact the British Red Cross Archives who hold the records of their WW1 VADs. The Red Cross came back to me with Rosaleen's WW1 service record and she worked at 54 Base Hospital in Wimereux, France until it was closed down at the end of the war.
I then wondered if Rosaleen had met Wilfred Owen and have begun to try to find out whether or not she was able to attend her brother Robert's wedding in January 1918 which Wilfred Owen also attended.
I have had two very good pieces of information from members of the Siegfried Sassoon Fellowship:
"Graves wrote to Sassoon on 22 April 1917 telling him that Rosaleen was nursing in Sassoon's hospital (4th London, Denmark Hill): 'I'll tell her to come and visit you.' In July 1918 Graves writote a letter suggesting Sassoon might get to see Rosaleen in Boulogne, where she was then nursing. See Paul O'Prey's excellent selected letters of Robert Graves 'In Broken Images'." From Charles Mundye of the Siegfried Sassoon Fellowship.
"When he wrote a rather delirious poem in a letter to Graves referring to "Lady Otterleen" Sassoon was actually conflating Lady Ottoline Morrell with Rosaleen Graves, who was indeed a nurse at the hospital in which Sassoon was treaded in 1918." From Deb Fisher of the Siegfried Sassoon Fellowship.
With grateful thanks to Sue Light and to Deb Fisher and Charles Mundye.