Try these.

http://www.castleuk.net/castle_lists_midlands/140/brandoncastle.htm

Only a little info here, but a picture is included.
If you scroll down to the map you'll see a triangular area bounded by Dyers lane, Coalpit lane and the B4455. Within this is Lammas Hill, from which Cromwells men sited their attack upon the castle.

http://www.newrider.com/Riding_Schools?schooldetails.php/17.html

This is the website for the Castle Hill Riding School on which the castle ruins are sited. You should find a contact name & telephone number of the owner. She should be able to help with more info, or at least point you in the direction of a local historian or two.

If memory serves, a dig was perfomed at the site early 1900's....findings at the Herbert Art Gallery/Museum, Coventry.

Hope this helps,

PeeJay.