Hi Tom, I am Martin Williams and I hail from Plymouth, England. My Mother is a Rowe by birth. The Rowes came from Calstock (over the river Tamar) east of Plymouth, which is up river from Saltash. I know from discussions with my dad who researches my Mothers roots, that recent ancestors of hers were Menheniots as well, and that they were in Calstock. I will ask my father for any info he has on the subject.
As far as the name Menheniot is concerned, I was also told that it is Celtic. The Old Cornish and Welsh languages had some similarity. Cornwall and Devon were actually the south part of Wales before they were annexed by the Saxons. This name appears to have originated before that time. It is also a place name and means "Place Of Old Stone". It is therefore a name composed of parts. I know from my understanding of Welsh (from being half Cornish and Welsh, a meaningless statement back in those days), that Hen is 'Old'. The Men, and Niot or Nett cover 'stone' and 'place' in some way. I think this is the Cornish coming out as these have no match in Welsh.