Hiya, it was a change to the REC conduct regulations, not actual law I think it's always been the legal responsibility of the employer to check, however in Oct 2010 the conduct regulations changed, here's a link to the news release
http://www.rec.uk.com/press/news/1237specifically it says
Agencies retain an obligation to check a candidate’s identity, obtain copies of qualifications and two references but only for those candidates working with vulnerable persons. There is no longer a need to carry out these checks for other roles where there is a legal requirement to have qualifications as this will be the responsibility of the client.
But REC advise (as far as I can tell) continuing to have the checking procedure in place as a contractual term between recruiter and hirer.