Old Hammersmith Pumping Station is now a Thames Water sewer workers depot. Its been split into two by a new floor added at ground level. Whats become the basement is original but all the pumps are gone.
You probably walked under it past the blocked off suction tunnels. You get a good view from the end of St. James Street - shown in old pic, its just accross the road from the new station., but is mostly hidden.
Your posts are fantastic, i'm loving these London sewer explores!