The River Westbourne rises in Hampstead Heath and was incorporated in Bazalgette's sewerage system as the Ranelagh Sewer. The Ranelagh Storm Relief Sewer was built along the course of the Westbourne to help alleviate flooding in 1862. I've been to this section a couple of times over the years but never ventured upstream to the Mid Level Sewer overflow.
After dropping down a lid in Knightsbridge I made my way upstream with TheVicar. We past The Egg connection, Sepentine infall and started the long walk upstream. There were a few side pipes with steps upto manholes, the best of which being a spiral staircase that climbed around one of the frequent ventilation columns.
Close to the overflow it fogged up a couple of times as mists rolled off the sewer.
A smaller tunnel connected in before the overflow coming from the old line of the Ranelagh Sewer. There was very little coming through here and wasn't pleasant wading through stagnent filth so didn't venture far.
I understand the Mid Level was built after at a lower elevation, causing the original tunnel to be altered to sump underneath. The normal level flow from this (and the other small sewers that connect later), is eventually intercepted by the Low Level far downstream.
The Middle Level Sewer No*1 enters in a large pipe around 14ft tall, this then shrinks right down after the overflow weir to around 6ft. Storm flows go down through the triple arched chamber and head off towards The Egg overflow, after which the pipe shrinks back to the older Ranelagh sewer.