[Koha] Editing a biblio record

We are "experimenting" in preparation for importing some 16,000 *book* entries from Excel spreadsheets into Koha (Koha 3.00.06 on Ubuntu Server 10.10) using MarcEdit. I have run into a snag editing a MARC biblio record using <http://koha-admin/cgi-bin/koha/cataloguing/addbiblio.pl?biblionumber=xxx>. We unfortunately got the "Title" of the Author (Sir Max Aitken) (100$c) into 700$a as a co-author. I can add|correct 100$c without difficulty, but cannot in any way delete or modify 700$a - I can highlight it with the mouse, but the keyboard is totally unresponsive. Any suggestions warmly welcomed - thanks. Paul