I might be eight years too late, but just in case:-
Devon FHS Index shows William Grinning baptised Lifton 21 July 1839. Parents John and Susannah living Tinhay. Free BMD records marriage of John Greening and Susanna Doidge March quarter 1839 Tavistock area. IGI has two Susanna Doidges - one baptised Mary Tavy 5 Jun 1817,parents John/Elizabeth. Other baptised Milton Abbot 5 Mar 1820, parents William/Sarah. As Doidge is/was a common name in West Devon, it is impossible to say if either of these were the same Susanna.