I was trying to track down information on Henry Shuckburgh Roots, who lived at 42 Russell Square in the 1830s and 1840s, and googling his name gave a link to your blog. I think I can help on the exact address of Sarah Green, the domestic servant. According to the 1844 list of members of the Royal College of Physicians, Dr Roots lived at 42 Russell Square. According to the 1841 Post Office Directory for London, No. 43 was occupied by James Christian Clement Bell, so John Foster’s house where Sarah Green lived must have been No. 41.
Hope that helps!'