From site: To earn your free night:
Join Marriott Rewards and be automatically registered to earn your free night certificate
Stay twice at participating hotels during your earning period*. Your earning period starts the day you enrol through the end of that month, plus an additional 120 days
Earn a free night at a category 1 – 5 hotel after your 2nd paid stay
E.g. very outer London hotels are £58-62 some nights. You can stay 1 night. Then another time, 1 night. Then another time, have your 1 free night.
You get 4-5 months to do these stays, but then a year to claim your free 1-night stay. Catagories of hotels can be seen http://www.marriott.co.uk/rewards/usepoints/hotelrew.mi