I'm not sure why, but I was slightly apprehensive about this one. It's known as Labyrinth or Serpent's Lair in Drainor speak. It's tricky to find drainors to chat with, so it's difficult to actually know precisely. I gave the chaps two options, and they went for this one. We met up in Hammersmith, as a resident of the borough, I was obviously last there!
I was feeling smug, as I'd turned up in wellie's rather than waders. This being a storm relief, we shouldn't encounter much in the way of deep water. The others pulled on their waders in a quiet passageway, and we headed off to a lid near the river. I'd lifted it the weekend before and saw a passage down, so was fairly confident. We pushed the stiff lid fully up, and Kev dropped down. He said it was a croucher, but doable. So we all dropped down. The pipe was round and had wet estuary type mud in it. We walked about 50m and came to a small local sewer going by and a dead end. Bum. So we turned around and exited. Gringz flexing his muscles with the stiffer than normal lid.

Due to the rather large lump in the gardens nearby, and having a map, I knew there was a large pipe nearby. I vaguely recalled an old lid i'd seen nearby, and on finding it, I popped down for a looksie. The shaft was went down around 10m, which would put it within the realms of a Storm Drain. On walking to the top gantry from the lid, a large mechanism was in place to lift a solid cover at the bottom of the shaft.

Looking down the shaft, it continues further down on the left from the landing lower down.

Two thirds of the way down was this flap covering a pipe.

Taken with the pipe above on the left in this photo. The raised pipe cover at the top centre, is the one the chains lead up to at the top of the shaft. The pipe leading East and off to the right in the photo is the one we explored. Seeing as obviously we couldn't explore any others. We could have gone down the pipe past the raised cover, but it's direction pointed to being an outfall chamber. Note the debris which clings to the ladder and gallery support, showing that the lower pipe completely fills at times of heavy flow.

Looking like the sort of view George Michael is used to on a Friday night, this was a croucher (one you can't walk upright in) tunnel we didn't have time to explore. The pipe opens out into the larger pipe we were walking in (just visible on the right). The main pipe we were walking in had a water level 3cms short of the top of my wellies, so progress was a little slow on my part.

The main pipe we were exploring had wooden panel sections on either side. I think there were about two for each side. Just behind the camera is the side pipe above.

Very impressive, in fact the grandest side exit i've seen. Sadly it lead only to a manhole.

The side pipe above sits on the right in this photo. Kev sets up a shot. The section just in front of him was another elaborate collection of ladders and platforms up to a manhole.

The section just in front of Kev in the photo above. Gringz was fannying about with shots above me.

Not far around the corner from the above is this huge pipe that leads into the pipe we were exploring. Gringz looks down at masses of small insects climbing over the nastyness. Just where he was looking were steps down into the thick sludge.

The large pipe referred to above. Progress would have been very slow up here, and chesties definitely the order of the day.

On and on we went, as I lagged behind in my wellies, muppet! Fancy thinking we'd find a nice empty storm drain. D'oh! Kev ahead.

Eventually we came to an effective dead end. Here two pipes cross each other. The pipe on the right ahead was on the same level as we were. Unfortunately the pipe on the left ahead (which connects with the pipe below) is at the same level as this tank for use of a word to describe the chamber. And that would involve chesties, as Kev saw most of his tripod disappear testing the depth.

The pipe that runs parrallel to the pipe we were in, and 'connects' with the pipe on the left above.

Not able to go on, we turned back and exited where we entered. Kev and Gringz finding a glamourous spot to change.

Further trips down this mental network to be continued...
This map shows the sprawling Hammersmith Storm relief in light blue.