We do not know very much about him but thanks to Maurice Hammond, who has just written to me, we know a little more.
Manno was apparently the lord of Ercé en Lamée, south of Rennes in Brittany. He was a powerful lord in his own right and fully able to assemble a fighting force to accompany him when invading England in 1066.
Maurice has also unearthed another story, that Manno had two sons, Hamo and Gauter, who contracted leprosy. Thankfully, according to this story, they were healed.
Somewhere between 1063 and 1084, when Abbot Bartholomew ruled the great Monastery of Marmoutier, Maino, the lord of Erce, came to him and craved him to descend to the little village of Guguen, some eight miles south of Dol, and heal his two sons, Hamo and Gauter, who were stricken with leprosy there. By the sign of the cross and a kiss of love from the venerable abbot, the youths arose miraculously cured. The father and grandfather, with their whole house and their retinue, made gifts of gratitude to the monastery among which Alan, the son of Floaud, conceded to the abbot and monks of Combourg whatsoever right he had in the church of Guguen. (From: The Isle of Bute in the olden time : with illustrations, maps, and plans, Vol 2, James King Hewiston)
This little anecdote may also fill in another gap in our knowledge. Manno's successor as Baron of Wolverton was Meinfelin, and as far as we can estimate came into the estates in 1114, presumably after the death of Manno. Meinfelin lived to 1155 so it has always appeared to me that there was a rising generation and that Meindelin was a grandson of Manno and not a son. Now Manno had at least two sons that we now know about, Hamo was probably the eldest. If he was cured of leprosy and went on to have children then it is possible that Meinfelin was his son. Meinfelin's son, probably the eldest, was named Hamon, or Hamo, and we can assume after his grandfather since this was conventional practice. It also occurs to me that Meinfelin might have been named after his grandfather, Maino and at some time acquired a nickname - "Maino le Felin" (Manno the Cat). The name stuck, as nicknames tended to in an age before surnames, and in time was contracted to Meinfelin.