Blockbuster are currently offering £1 per night fee if your DVD is overdue. But if you borrow another DVD when you return the first one
they cancel the overdue up to the cost of the new hire. So keep the DVD for an extra four nights and borrow another DVD when you return it and your £4 hire fee cancels out the overdue. Four extra nights for free! This was in my local Blockbusters in Edinburgh, hopefully it's a national policy.