David Barclay was one of the 35 members of Meeting for Suffering Committee on the Slave Trade that first met in 1783. 

Committee membership:

Meeting for Suffering Committee on the Slave Trade (1783-1792):
Jacob Agar; David Barclay (1729-1809); Adey Bellamy (1739 – 1810); James Beesley; Rich... From NEN Gallery