Thank you so much for your reply Deborah
I did look at that marriage, but Mary Forster's parents were Robert and Thomasine, and neither name features in the many children of Rebecca Light. However, there is a marriage of a Philip Leight to a Mary Roe on 8 April 1785, at Newton Ferrers, and her parents may be Samuel Roe and Mary - interestingly, there are two of Rebecca's children that I can't place in any naming pattern, those being William and Samuel. Might be a clue? At the marriage of Rebecca Light to John Garland Williams, one of the witnesses was called Sampson Light; I have traced him through the census, and he was born 2 years before Rebecca; his second marriage states that his father was Philip Light, Sailor. Frustratingly, I cannot find a baptism for him! Am going around in circles at the moment, with no definitive "proof" of which Mary is the right one.