Until 3 March 2014 people who received Housing Benefit continuously since 1996 were exempt from the under occupation penalty (the 'bedroom tax'). The government changed legislation so that since 3 March 2014 such people are no longer exempt and the under occupation penalty is applied in the normal way.
This means you are affected by the under occupation penalty (bedroom tax) from 3 March 2014 even if you have been claiming Housing Benefit continuously at the same address since before 1 January 1996.
However, the change is not back-dated and if you had the under occupation penalty applied you could get money back for the period 1 April 2013 to 3 March 2014.
In order to get money back for this period you must not have had a break in your claim or lived anywhere except at your current address in that time (except for any period where a fire, flood, explosion or natural catastrophe meant it was not possible to live in the property). You may also qualify if you have lived at your current property since before 1 January 1996, even if you were not the tenant, if you inherited your tenancy from a family member.
If you have had money taken off your Housing Benefit because you have a spare room and have a pre-1996 tenancy you could get money back for the period 1 April 2013 to 3 March 2014. You should contact your local council or housing officer to make a claim.