Thornhill Hundred, Wiltshire is in Wiltshire.
Ermine Street 41b travels from Speen, Newbury [Map] through Baydon, Wiltshire [Map], Wanborough, Wiltshire [Map], Blunsdon St Andrew, Wiltshire [Map] to Cirencester, Gloucestershire [Map]. Whilst Margery's scheme doesn't say so, the road appears to continues from Cirencester, Gloucestershire [Map] to Gloucester [Map].