In order to instruct on a Civil Light Aircraft you need a Civil FI rating. The Seminar is part of the process of revalidating or renewing such a rating if you already have one. If it has expired by more that 5 years you will need to do complete both the Seminar and Test and submit the application to the CAA for reissue.
See LASORS Section H
http://www.caa.co.uk/docs/175/SECTION%20H%20.pdf