He moved in 1747 to succeed John Hill (1711–1746) as pastor at the Three Cranes meeting-house in Fruiterers Alley, Thames Street, London.
Sandeman had published (1757) a series of Letters dealing with the Dialogues between Theron and Aspasio (1755), by James Hervey (1714–1758).
This led (August 1758) to rumours of his unsoundness; his discourses at Pinners' Hall gave offence, and he was excluded from the lectureship in 1759 by forty-four votes to one, Dr. John Conder being chosen to succeed him on 3 Oct.
Disputes then arose about possession of church property, and a lawsuit was begun (1761) by Pike for recovery of an endowment of £12 a year.
At length he resigned his charge (14 December 1765), left the independents, and became a member of the Sandemanian church in Bull and Mouth Street, St. Martin's Le Grand.