Sorry for the late addition here, but one thing to bear in mind with the new season underway is that the football affects 313 allocations on the east side. Last time I was down there on a matchday Saturday, a few years ago now, there were two 6 car 313 sets on the Lewes service but all Seaford workings were 377s in multiple. I think non-matchday Saturday services to Seaford were/are normally 313 but often one or two 377s deputised.
Whilst we're on the subject, yesterday actually marked the 45th anniversary of 313 operation. On 16/08/76, they began life on a 'preview service' from Old Street to Drayton Park. Most, if not all, 313s used that day are amongst those surviving with Southern today.