Phil, I am making what could be wildly inappropriate assumptions from reading, OFFICIAL HISTORY OF THE CANADIAN MEDICAL SERVICES 1939-1945, but it appears to me that No. 21 C.G.H. was not accepting casualties until it went to the continent following D-Day, to Mesnieres en Bray, near Dieppe, to be precise. See quotes below.
From page 235:
Meanwhile, in the United Kingdom the role of Nos. 16, 20, and 21 Canadian General Hospitals, which were awaiting transfer to the Continent, was the subject of some discussion. It was felt that they should be prepared to receive casualties from the Continent should the static hospitals be unable to cope with all of them. But it was pointed out that as of 18 July Canadian static hospitals in the United Kingdom had 5000 empty beds and that if all the British casualties in these hospitals could be moved out the number of beds available would be greatly increased. Arrangements were made with the War Office to provide emergency hospital accommodation for the three hospitals should it be necessary. By 12 August operations in Europe had gone so well that the Deputy Quartermaster General, Canadian Military Headquarters, was able to advise: "it is deemed that the emergency provided for will not now arise." The matter was then dropped.
From page 265:
ARMY AND BASE UNITS
The lack of movement during October as compared with August and September greatly simplified the problem of evacuation within army formations and allowed the larger lines of communication units to be brought again within effective range of the fighting front. At 6 October the following general hospitals were open:
Antwerp Area No. 9 British (600 beds)
No. 9 Canadian (600 beds)
No. 30 British (600 beds)
No. 6 Canadian (200 beds) St. Andre
No. 12 Canadian (1200 beds) St. Omer
No. 16 Canadian (600 beds)
Behind the First Canadian Army there were two Canadian general hospitals at Bayeux (Nos. 2 and 10), one at Martigny, (No. 7), one at Mesnieres en Bray, near Dieppe (No. 21).
From page 270:
Nos. 2, 10, and 12 Canadian General Hospitals remained respectively in Ghent, Turnhout, and St. Andre until disbanded, the first two in September, the third in November 1945. No. 20 moved in March 1945 from Antwerp to Turnhout, remaining there until disbanded in September 1945. This unit had arrived from England in December 1944 and had been operating at partial capacity and under great difficulty because of rocket attacks. No. 21 Canadian General Hospital moved from Mesniere en Bray to St. Omer on 9 January 1945 and remained there until proceeding to Turnhout where it was disbanded on 10 September 1945.
There are no other references, available to me, which give locations for No. 21 C.G.H.
I conclude, from the above, that No. 21 Canadian General Hospital, could have been at Bramshott, not taking casualties, and, therefore not mentioned in any of the histories other than above, and was eventually replaced by No. 22 C.G.H. as Lloyd Jackson states. (page 206 of Official History of the Canadian Army in the Second World War, Vol. Six Years of War: http://www.cmp-cpm.forces.gc.ca/dhh-dhp
... xyrs_e.pdf, confirms No. 22 location as Bramshott).
I believe the only way to come to any definititive answer would be to consult the unit war diaries, or you could try contacting the Museum of Military Medicine (UK) [https://museumofmilitarymedicine.org.uk/