There is Anne Stone Age 6 with the family in 1871 as a daughter.
Could she be another grand daughter and the daughter of Susan?
The mind boggles 
Annie Stone born Jun 1864 Yeovil has no MMN on the GRO index
I'd got myself so confused over all the various perturbations of the family, that I'd forgotten about there being an Ann Stone present in John & Mary Stone's household in 1871.
So again it's another example of two children on the same census return being described as daughter and grandson when they are both, if fact, grandchildren.
Another piece of the jigsaw falls into place. If Annie is in London from 1881 onwards, it presumably makes her prime suspect as the mother of Maud.