When first stepping foot in Crypt one could be forgiven to thinking it is an anomaly in the system. Seemingly the purpose of this chamber is to act merely as confluence between two egg shaped sewers, the largest being the Putney Extension. These travel either side of a lower central channel in an elaborate arched chamber, one of the best I have seen.
The flows meet at the end of the chamber where they drop into a lower pipe. Thanks to some excellent research from sub-urban, it looks like the chamber used to serve as an overflow. The two sewers would have continued through the chamber and onto another feature. The chamber ends have been bricked up so the flow now continues down the old overflow.
Big shouts to Nckt, Eotech, Winchester & Ali.