[Koha] Editing a biblio record

[Paul]

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