Is it a Milbro (i.e. British) Diana Model 2 or an MGR (i.e. German) Diana Model 2?
If the former, then it will date to 1945-1965, and if you are lucky there could be a date stamp impressed on the heel of the grip.
If the latter then it will date to 1938-1940. These are much rarer.
The fact that it is nickelled adds to the rarity, whichever model you have.
(More detailed information can be found in the Encyclopedia of Spring Air Pistols, Second edn.)
Cheers,
John