Matt, this is the info from the letter we were given:

"Guest and Ticket holder Access: Important Information to pass on…
There are special arrangements for Southbank Centre on New Year's Eve as the area around Royal Festival Hall has restricted access from approximately 8pm. After that time police cordons will be in place and you will only be able to get access to our site if you show your ticket. At approx 10pm the cordon will be sealed, after which no-one will be allowed access to the site. This is in the
hands of the police and if there is overcrowding this could happen earlier. Please try to get all guests on site before 8pm when the crowding begins to back up."

There's a lot going on inside the RFH besides the mainstage stuff, including a full Magpie's Nest/Folklahoma programme and song and instrumental sessions, but it looks like access is restricted to ticket-holders.