Yes, there was a churchman by that name. Not a Saint, but at least blessed
Manfred. He lived in Northern Italy in the 14th century. He has a name day in the Catholic Church, so this probably had some impact at least on the continent. He is not mentioned though in my Dictionary of Saints (Oxford). It's
Manfredo in Italy.
As far as I know (I think I read this in Hanks & Hodges' Oxford Dictionary of First Names), the name
Manfred was introduced to England by the Normans. But I guess, it never really got popular and in the 20th century, as you say, was looked upon as
German rather than Germanic. I never met one
Manfred, when I was in England, so I assume, the name is quite rare.
Thanks for your help!