According to the Indie London website, tickets for the Apollo concert are available from the Ticket Hotline, on 0870 606 3400, as well as Stargreen (020 7734 8932) and usual agencies. Sales start on September 10 and prices might be around £80 (plus credit card and booking fees).
UPDATE: According to Stargreen, the information should be going up on their website
by Thursday latest. Tickets on sale from 9 a.m., September 10.