I found a marriage 18/7/1798 at Milverton of James Musgrave and Betty Bond, so this is quite possibly this is the Betty Musgrave buried 1859 aged 80.
I keep hoping that the gap in the Milverton burial register between Feb 1860 and March 1890 will suddently fill in and I might find her .. I hope when I get to Taunton RO that they might those records.