The family tradition about the Shirley/Ferrers connection is what prompted me to buy a copy of "Stemmata Shirleiana" but nothing in it casts a glimmer upon any connection that I can spot. I have been off on one or two wild goose chases but no luck so far. The "connection" is also rather muddied by the use of Shirley as a first name amongst some of my Sewell recent ancestors - I think that a co-incidence.
I'll e-mail you a scan of the first tree in "Stemmata Shirleiana" so look out for a pdf in your junk tray.
John