Jancis, thank you for starting this thread. We may not crack the nut, but at least we can all contribute to the nutcracker suite.
According to the Alz Soc - “The Mental Health Act (MHA) is designed to protect the rights of people in England and Wales who are assessed as having a 'mental disorder'.
This is a general term used in the act to describe any disorder or disability of the mind, including dementia.” http://www.alzheimers.org.uk/site/scripts/documents_info.php?documentID=117
As the Mental Health Act 1983 covers people with the mental disorder of dementia, and as Sectioning under the Mental Health Act 1983 covers people with dementia, and as Section 117 ‘aftercare’ covers people with dementia, dementia comes under the Mental Health Act for many problems that people with dementia face.
The Mental Capacity Act 2005 covers not only people with dementia, but also people who have mental health problems resulting in a lack of mental capacity. The causes of mental incapacity are wide-ranging and include dementia. Dementia is not the only cause of incapacity, as far as the MCA 2005 is concerned. Emotional disturbance is also there.
http://www.patient.co.uk/doctor/Mental-Capacity-Act.htm
I didn't realise that MIND thought differently about dementia - but I will try to find out more about MIND's reasoning.