Just got an email about this, worth having a look if you can! £40 gift voucher if you were planning to go away before December and all you have to do is leave a review!
Book your next hotel on the TripAdvisor mobile app using a valid UK billing address (minimum £100 spend before taxes and fees). Be sure to complete your stay, then submit your review by 30 November 2016. Once your review is published and we've confirmed your stay, we'll email you a code worth £40 for use on Amazon.co.uk. Limit of one (1) Amazon.co.uk Gift Card per person during the promotional period.
Chanchi32 has pointed out:
'Only users who have previously booked a hotel on TripAdvisor through "Book on TripAdvisor". Previous booking(s) must have been made with the same email address used for the booking qualifying for the TripAdvisor app £40 Amazon.co.uk offer. 1st time bookings do not qualify for the TripAdvisor app £40 Amazon.co.uk offer'