ATS was considered part of the ARMY.
Service records are still held by MOD.
Advice and forms for obtaining personel service records
HEREIn Brief:- If the person is still alive then only they can obtain their records (without charge), their spouse, (or parents if no spouse) can obtain the records also without charge but will need to supply a copy of the person's Death Cert.
For anyone else it will cost £30, a copy of the Death Cert. plus patience.
There is a restriction on the amount of information supplied to non NOK for 25 yrs after date of death.