It was always my understanding that the annual electoral roll document that each household filled in and submitted to the local council was confidential. I had a rude awakening in the 1980s when a local councillor said that wasn't the case and that they sold the information - which solved the puzzle of why sales bumph was addressed to us and weren't just addressed to "The Householders".
No at that time the full electoral register was available to anyone who wished to purchase a copy, it was not until 2001 that the open register was produced in answer to a High Court ruling in November 2001.
The government like you to believe the confidentiality clauses were put on the census forms to prevent public access but in reality they were put there to prevent other government departments accessing the data, there was a big fuss in 1871 after census officials in London divulged the names and addresses of all children 3-13 and their parents to the London School Board to help enforce compulsory education.
The early census text stated 1861, 1871, 1881
“The facts will be published in General Abstracts only, and strict care will be taken that the returns are not used for the gratification of curiosity.”
The 1891 added
“The facts will be published in General Abstracts only, and strict care will be taken that the returns are not used for the gratification of curiosity, or for other purposes than those of this Census”
The text then stayed the same until the 1911 made it crystal clear
“1911 "The contents of the Schedule will be treated as strictly confidential".
The following also appeared on the reverse of the Schedule:
"The contents of the Schedule will be treated as confidential. Strict care will be taken that no information is disclosed with regard to individual persons. The returns are not to be used for proof of age, as in connection with Old Age Pensions, or for any other purpose than the preparation of Statistical Tables”
The 1921 and 1951 returned to "STRICTLY CONFIDENTIAL"
With the 1961 stating
"CONFIDENTIALITY--No information about any individual person, family or dwelling, will be given to anyone not employed on the Census".
This was again changed for the 1971 census
"The information you give on the form will be treated as CONFIDENTIAL and used only for compiling statistics. No information about named individuals will be passed to any other Government Department or any other authority or person.”
Then in 1981 the Registrar General added
"Your replies will be treated in STRICT CONFIDENCE. They will be used to produce statistics but your name and address will not be fed into the census computer. After the census, the forms will be locked away for 100 years before they are passed to the Public Record Office".
He later had to apologise to Parliament for exceeding his authority by adding that text.
Cheers
Guy
PS If you read the Census Act 1920 as amended by the Census Confidentiality Act you will see that despite what the National Archives claim it is actually unlawful for any further census to be released without a change in law
http://www.rootschat.com/links/0wkf/