Instructions:From LoveFilm insert:
£5 off your first purchase at Waterstones.com when you spend £25 or more exclusive of online postage and packaging. Enter claim code in 'Promotional code' box at the checkout
One discount code per transaction and each code may only be used once per customer.
Terms and Conditions themselves do not explicitly state this can only be used for your first purchase. As ever, free postage above £15 or to store.