The Home Office has collected and published information on the number of persons detained under section 136 of the Mental Health Act 1983, including information on the ethnicity of those being detained, since April 2015.
Data for 2015/16 were requested on a voluntary basis only. Information was provided by 15 forces, and data on section 136 detentions were published as ‘experimental statistics’ in Annex B of the ‘Police powers, England and Wales’ statistical bulletin, in October 2016: https://www.gov.uk/government/statistics/police-powers-and-procedures-england-and-wales-year-ending-31-march-2016
The data covering the 2016/17 time period is due to be published in the next edition of the ‘Police powers and procedures, England and Wales’ statistical bulletin, which is scheduled for release on Thursday 26th October, and will be available here: https://www.gov.uk/government/collections/police-powers-and-procedures-england-and-wales
Information on the number of detentions under section 135 of the Mental Health Act 1983 will be published as ‘experimental statistics’ in Annex A of the next edition of the ‘Police powers and procedures, England and Wales’ statistical bulletin.
Prior to data being collected by the Home Office, the National Police Chiefs’ Council collected information on the number of detentions under section 136 of the Mental Health Act. Data covering the 2015/16 time period, broken down by police force area, are available here: http://www.npcc.police.uk/documents/S136%20Data%202015%2016.pdf
Previous year’s data can be found here: http://www.npcc.police.uk/documents/edhr/2015/Section%20136%20MHA%20201415%20Data.pdf