It was weird. I had an alert from SDH yesterday (as I'm on the mailing list) to say that I could book pre(general)sale tickets from 11.00 this morning (Friends of SDH got in even earlier). I logged on about 10.45 and managed to book then for some reason.
Just looked at the plan and there are at this moment about 60 £63 seats left - but they may go before the morning of course.
Why don't you try joining the mailing list and see if you can get in that way. Don't know if that'll work but it's worth a punt.