I was still waiting on some vouchers (from before Christmas, so it was approaching 8 weeks and they do say it may take 6-8 weeks) so I contacted coke via the online form on the cokeunleashed site
I got a very friendly reply stating that there was a backlog from Christmas time but that they had checked and my vouchers would be on the way soon, and that they had credited my account with 30 points for the inconvenience
I recommend emailing if it's been a few weeks and you haven't received a voucher!