Thanks for the replies! I'll actually be there during the tournament (arriving a week from tomorrow) so I've emailed our hotel concierge as a first step. Hopefully they'll be able to help.

Good to know that if this fails we might be able to get tickets once there. We're not so picky about seats and figured we'd go midweek -- we're going more for the cultural experience and to learn more about sumo; we're not huge fans (yet)!