the circle is unreserved seating, but everyone queues together, you just go separate ways when in the door.
There are plenty of seats, all with decent views as it is quite tiered.
As for time, for a Stray Cats gig i got there 5ish, not sure if this gig has the same draw as a Cats gig, so it`s anyone`s guess....
BUT you will get a seat, as those with downstairs tickets can`t come upstairs, and vice versa.
Dave