You will need to fly a SEP revalidation LPC before the date of expiry, or, if the rating expires before you get a chance to do the LPC, then it becomes a SEP renewal LST. Do it within 5 years of the expiry date and the Examiner can sign you up there and then - otherwise you will need to send your paperwork off to the CAA.
All this should have been covered in your Air Law study.....
The LST/LPC does now include a mandatory navigation element, the content of which will depend on your recent flying activity and is at the Examiner's discretion.