Not sure if I am reading this right but the W/R/A was brought in in 2007 according to the first link
It was however the current Gov who set out the descriptors (second link) which ATOS are supposedly following
http://en.wikipedia.org/wiki/Welfare_Reform_Act_2007http://www.publications.parliament.uk/pa/jt201012/jtselect/jtrights/233/23305.htmI have been sent this information below.
ESA was first introduced by the Bliar government in 2008, using input in developing the descriptors from different sources such as ATOS, and UNUM a disreputable US insurance company.
The descriptors are actually part of the ESA Regs and Legislation set out in law, they are not a separate entity.
ATOS developed the computer software used (LiMA) to carry out the assessments.
In March 2011 the original 2008 descriptors were changed to include the "imaginary wheelchair" rather than just a walking test. This was obviously done by our current coalition government who also decided to push ahead with the programme to migrate all IB, IS and SDA claimants over to ESA (March 2011 To April 2014)
The descriptors were changed again on 28/01/13, by the present coalition.