Quite simply return the book. If your loans are more than 5 weeks overdue they become Billed Items. You will be issued with a replacement bill for the items you have on loan and your library account and network account will be blocked.
As soon as you return your loans the bill will be lifted and replaced with a fine. Your library and network accounts will be restored. So finally you just need to pay your fines.
All this information is on LRWeb: http://lrweb.beds.ac.uk/libraryservices/availableservices/borrowing/fines