At Jewish East End we found photos of the inside of this synagogue and "In 1887 local MP, Samuel Montagu, fearing for the safety of the users of these cramped spaces, founded the Federation of Synagogues to amalgamate them into larger, safer premises - hence the term ‘Great’ in the title of many of the Federation’s synagogues, and hence the foundation of Fieldgate Street Great Synagogue in 1899."
This is one of the few remaining synagogues in the East End, and it is struggling. On another one of Jewish East End's pages we found a link to a professional 11 minute Jewish film illustrating the problem in a lovely way, with Warren Mitchell.
More photos at Spitalfields Life where we learn that it was damaged in WW2, rebuilt and reopened in 1959. It closed for regular services in 2007 and was sold to the (surrounding) East London Mosque in summer 2015.