Guy; Just point me, when and where to sign 
It is far better to write to an MP or Cabinet Minister via the House of Commons.
E.G. if a Cabinet Minister the letter should be addressed -
Rt. Hon. Ian Duncan Smith MP.
House of Commons
London
SW1A 0AA
If an MP simply - Mr. Bloggs MP
An example letter could be-
Dear Mr. Smith,
In view of the current financial position of the UK and in view of the fact that there was no commitment to the public in 1921 that the census would be withheld for 100 years I wish to ask that the 1921 census be released now.
The result of this would bring economic benefits to the UK at a time when it is most needed. It could also provide much needed employment.
The economic benefit may be shown by a Hansard reply of Mr Carmichael where he states
http://www.publications.parliament.uk/pa/cm200304/cmhansrd/vo040329/debtext/40329-44.htm“According to the National Audit Office report on the release of the 1901 census, "Unlocking the Past: the 1901 Census Online", the internet site allowing access to the England and Wales records generated revenues of £4.5 million by 31 October 2003.”
The 1901 census was released online in November 2002 (released initially January 2002 but withdrawn until November after the site crashed due to high demand) therefore that sum must be for approximately one year or less.
The figure may seem modest in national terms but in times of financial stress every form of income is a valuable asset.
The facts about withholding the census are also discussed in the above Hansard reply and I feel that in these times of open government and honesty in parliament the closure period should be urgently re-examined as the legislation that forbids the release of the 1921 census was not enacted until 1966 (Lord Chancellor’s Instrument number 12) and later under the Census (Confidentiality) Act 1991.
As these two pieces of legislation are retrospective they did not influence the electorate when they filled out the census schedules therefore they could be amended without breaching the trust of the electorate.
Yours faithfully
If MPs and cabinet Ministers get hundreds or even better thousands of letters coming in day after day they sit up and take notice.
Do not forget every MP is looking for ways to avoid making cuts in their department.
The 1921 census provides the means to avoid cuts, create income and provide jobs.
That is exactly what this government claim they are endeavouring to do, it therefore ticks every box.
Cheers
Guy