I had a word with the editor of history.uk.com, he said you should ask college of arms for any pre 1400 historical info such as earldoms.

The College of Arms
Queen Victoria Street
London EC4V 4BT
Tel: +44 (0) 20 7248 2762
The College is open each weekday between 10 a.m. and 4 p.m
Web site: www.college-of-arms.gov.uk