Robert of Gloucester sounds interesting. His second son being a bishop certainly doesnt rule him out. The idea that they were celibate has little foundation when you look at their lives in detail. Most bishops had illegitimate children, and the church even had ways of dealing with them to enable them to become legitimized sufficiantly so that they could become clerics themselves.
Im trying to get my Father involved, as he is good at this sort of thing. Expect there will be lots of deadends, but who knows.