OK thanks, I spent the day down there yesterday and it turned out there's an hourly Ports Hbr to Brighton, which were all 313s, or all the ones I observed anyway. Also an hourly Ports/SS LL to Littlehampton, also 313. The Southampton to Brighton and the Ports Hbr to Victoria were both 377s.
Naturally I chose to go Ports Hbr to Gatwick, then down the main to Brighton. I wasn't going to sit on a 313 for almost an hour and a half !
(I could've gone up to Havant or somewhere and picked up a Southampton to Brighton, but it wasn't a good connection)