This doesn't strictly come under the heading of censuses so, Mel, please feel free to move it if you think there's a better spot for it.
The 1941 census didn't happen, due to the war. However there was a similar thing in 1939 when everyone was required to register for a National Identity card. Apparently some women didn't register their sons, in an effort to prevent them being called up if war broke out. However, when they realised the card was needed to get a ration book they came forward and 'declared' them.
The card details are available, but only in respect of people known to be dead a the time of application. And the fee is £42 so it's not what you'd do lightly.
http://www.1911census.org.uk/1939.htm