Hi Martin
If you look on
http://www.rafweb.org/Menu.htm#Squadrons and look up both 11 Sqn and 21 Sqn it says that both Sqns were allocated the code YH- from Sept 1939. For 11 Sqn it lasted until 1943 and for 21 Sqn until 1947. The site is usually very reliable but I agree that the double use is not the norm. It may be significant that Sept 1943 was the date when 11 Sqn converted to Hurricanes and went East to serve in the Indian Sub-Continent and Burma having been there from the start of the war and moved into the Desert/Greek theatre after May '40
Regards
Dick