Hello,
I've finally got back to dierbo, now that I'm under the main floor, after going down the tunnel, I can't turn the switch at the end, it won't budge. Is this a glitch, or what have I not done? Anybody? Thanks

Speaking from memory.

I beleive you need to activate a lever after the ride back up the great shaft elevator. It must be in one of the side room (corridor). If I remember correctly it will unfold a steel ladder (to the last room/pedestal to Derebo), in the lower section corridor and will have unlocked the lever to activate the shaft's fan.