The rules regarding access to medical records (especially those relating to mental health) can be confusing. In general, adult patient records are closed for 75 years following the date of death, while those of minors are closed for 100 years. If it can't be proven when someone died, then records will remain closed for 100 years.
Some archives have procedures in place for accessing records within those times, and will make decisions on a case-by-case basis. You'll need to see what procedures NHS Tayside have in place for their records which are held by the University of Dundee Archives. The Archives will be able to advise you on how to go about requesting access.