Thanks, Suey!
The army would be an interesting connection as her son-in-law was in the army, and thus her daughter was dragged around various parts of the UK with him!
As has been said, I just need to find a baptism for a Ruth Horton in Lewes c.1765...but none seem forthcoming from FamilySearch. I can check on Ancestry at lunchtime in the local library, but if anyone can get there first and let me know if it's worthwhile or not, that would be hugely appreciated!