Dear Sue,
I am descended from Isabella De Clare and her husband William Marshall. Their sons inherited the title of Earl of Pembroke and 5 of their sons in succession from oldest to youngest held that title all the way to the 9th Earl of Pembroke.
I find no record of the title being passed to the Herbert or Handebo families, so I suspect that whomever is in Australia claiming that descent is in fact incorrect.
I am interested in exploring this more, if you have any more information I would love to hear it.
Also, women did in fact inherit titles, that is a misgiving that they did not. Isabella was well known as the Countess of Pembroke before getting married to William Marshall.
William Marshal in fact was the Marshall of Pembroke that he inherited from his Father John Fitzgilbert. His grandfather was the Sheriff of Wiltshire. Surnames in that time sometimes reflected the Title carried by the family.
You may find this information in:
The Complete Peerage vol.X,pp.358-364
I hope this helps!
-Darren