i also get the "There is an outstanding fee debt against your record..." message!
i think i got the answer as to why though: on this yellow paper i got with the Law welcome pack it tells me:
In order to complete on-line registration you will be required to have:
1. Sent back your Acceptance Form, passport sized photograph(s), and proof of any outstanding conditions relating to your offer.
2. Paid, or made arrangements to pay, your tuition fees for 2006/7.
did you manage to access the site mark?