The IMHA service was introduced by the Mental Health Act 2007, coming into force on 3/11/10 in Wales and 1/4/09 in England.
DH "Independent mental health advocates" page
NIMHE: Independent Mental Health Advocacy: Guidance for Commissioners - published 18/12/08
Mental Health Act 1983 IMHA Guidance: Commissioning the IMHA service - Welsh Assembly Government
Rezina Hakim and Tom Pollard, 'Briefing Paper 3: Independent Mental Health Advocacy' (Mental Health Alliance, dated February 2011, published 17/3/11)