Coloured diagram of the internal roof structure from the Planning Statement. It's not exactly the same but close enough to see what's going on.
From what's on site it looks like they'll use the two big cranes to lift the central secton of the main roof truss which has 9 structural bays (coloured orange).
The two support towers (blue) are almost complete and somewhere on site there will be two side truss sections (purple) that connect the central section to the towers. I think I saw them in one of Mr Drone's videos by the Portacabins in Stanley Park.
They've erected three central sections of seating so that they can connect four* trusses (green) to stabilise the whole thing.
Once that's up the rest can be erected with smaller cranes and the central floor sections can be filled in.
*edit: three structural bays = four trusses