In reply to steve.aisthorpe:
Probably not what you want to hear, sorry.
No flexibility that I’ve seen between the two blocks. Only flexibility is you don’t have to arrive exactly at the time of the actual session starting time you book (i.e. you can book say a 10am start but arrive 11 or after).*
Unless it’s changed since last time I was there before Christmas, you would have to book a session starting in the each of the two main blocks; members even have still to leave the arena for the half hour of cleaning that takes place between the two main blocks per day.
* BTW I assume you know that you have to be “verified” to climb/boulder and this has to be as I understand it since the new system came in last year. New registration was required with the new system. If not verified you need to arrive for one of the two only sessions per day - 10 or 4pm.