He's my GGGGG Grandfather too and I've never managed to work out where he came from. There is a tantalising mention in the Poor Law records of a James Rainbow being removed to his parish of origin (Radwell in Herts) at about the right time to potentially be our forebear reduced to a doddery and indigent state (I transcribed it and can look it up if you want) but I haven't yet checked out the Radwell parish registers to see if his birth is recorded (and it's not on the IGI).
I assume you're Sally Tassie's son? You're on my database.... Big Brother is everywhere!
I do have Mary Walsom's forebears going back a while, but I've already given those to your mother!