There is already another post on HUKD with this offer, but I thought the lack of awareness on how to get the voucher without a purchase warranted another thread. All credit goes to the OP of that thread - aishaukhan, and the other members who commented on there.
How to Claim:
1. Vote for your favourite comedy movie from the top 5 list
2. Enter the unique code shown on the neck of your promotional YAZOO bottle (Enter ANY random code using a letter, followed by 3 numbers, or vice-versa, eg. L148 or 964G - this is not checked)
3. Select a cinema of your choice from the listings and print the completed voucher
NOTE: There are very few main cinemas participating, but there seems to be one available in each city/location.