Please can someone advise on how long we need to keep absence records?

I have just discovered that we have 3 different ways that the records have been dealt with over the last decade:

1) we have folders in academic years where every single absence sheet is filed (sick certificates, return to work forms, absence request forms for other reasons etc). Apparently for quite a while, the original doctors certificates were sent to the borough, so we took copies. These folders date from 2009 - 2013

2) Each staff member has a personnel folder (electronic and paper) - all their absences (some from up to over 10 years ago) are filed in their personnel folders electronically and hard copy.

We have absence records for leavers and current staff. Not all sickness related.

How long do we need to keep them for before we are able to destroy them?

Thank so much for any advice. 

Many thanks

Jenny