https://www.seagullstickets.com/default.aspx?qit&q=9fc51262-dec0-420b-b4af-4a50ce7596a8&p=8709c549-9599-4a77-a970-ad9f5a97733b&ts=1461222888&c=bhafc&e=bhafcq&rt=Safetynet&h=5276262a8583841446bba08c7f4a02b6
According to their website it's sold out.I'm not sure if you can get tickets on the day...
You need to go on the Brighton website.You can see exactly for yourself what tickets are available for which stands/seats.You should always find a seat.