I had a similar experience with a book offer they had a few years ago - the cheque was cashed, but the books never showed up. I tried phoning, sending letters, all to no avail. However, it worked out for me because I had also sent off a cheque for a Royal Doulton dinner service, which was for a similar amount. I got the dinner service, but the cheque was never cashed.
Check a copy of the newspaper for their contact details - it'll be buried in there somewhere, probably in very small print. As Maread said, I wouldn't let that go, given the amount of money involved.