Hi,
The unit Operational Record Book is held at The National Archives
AIR 29/760
4 Medical Receiving Station; formed at Helwan (Egypt) April 1940, renamed 21 MRS May 1940, Western Desert; renamed 21 Mobile Field Hospital May 1943 and moved to Sicily July 1943, moved to Italy by November 1943, moved to Klagenfurt (Austria) May 1945; disbanded June 1945 (MFH).
http://www.nationalarchives.gov.uk/catalogue/search.asp
Regards
Ross
Restorer of Canberra PR.9 XH175 and 8249 (front runner at the moment) Anson Mk.II