The DfE have not given definitive guidance regarding how far to go back to retrospectively acquire the second reference for staff who may not have one.
At an INSET I recently attended run by Andrew Hall, Safeguarding Expert, he suggested that many schools he is aware of have chosen not to seek a 2nd reference for staff who were employed over 5 years ago. However, dependent on what Ofsted Inspector you get, their views on this can vary greatly!
He advised that schools should follow their LA guidance, which you will find below.
In light of what we know about recent historical sex offences and following the information we have been given from HMIs in the School Improvement Team, who have had experience of OFSTED Inspectors being adamant that there are 2 references in place, as a local authority we would like all schools to acquire a 2nd reference for all their employees, including existing staff.
The guidance below explains how to go about acquiring these.
DBS Risk Assessment Template
Historical References Guidance