For a while I've being trying to get back to photograph this feature after a quick reccie with Nckt. One rainy day finally gave the right conditions to slip in unnoticed.
The manhole shaft leads down inbetween the A & B outfall sewers. At the bottom is a 6ft pipe that connects both trunks to allow the flow from one to enter the other when flow levels are high. This ensures neither trunk is overly loaded compared to the other and the system is as balanced as possible. A penstock can be lowered from an alcove above to block off the connector.
When I first went down the flow was a mere trickle. This time it had been (and was) raining, the balancer was clearly doing it's job as a torrent was flowing through the connecting tunnel.