During PCA examinations, what input can the person/carer/adviser accompanying the subject have into the process?...I'm interested in mental health subjects in particular

I always check first to see that the client has mild to moderate mental health problems first. This is because if they have a severe mental health problem, they should be exempt from the PCA.
For the definition of severe mental health see the Incapacity Benefit Handbook for Medical Services Doctors and Commisssioners file CIB/3328/1998. (Perhaps Ken can provide a link).

That commentary indicated that a severe mental illness can be evidenced by the presence of, living in sheltered or a residential facility, or day care once a week, or care at home with intervention once a week by a CPN or Social Worker, or long term medication with either depot injections or mood changing drugs.

If the client is suffering mild to moderate mental health problems (Usually anxiety/depression), then I show them, and the person accompanying them to the medical, the mental health descriptors to identify which ones apply to the client. They can then make sure the medical examiner is fully aware of the issues involved.

aware of issues on pca/severe mh etc...thanx anyway...the query was more about the rights of the accompanying person to speak on behalf of the person etc

Check out page 38 onwards of the guidance for the doctors on people who are accompanied...
http://www.dwp.gov.uk/medical/ibh/ibh.pdf