I had this problem with my previous Fiesta. At first the joint at the bottom of the steering column was greased - that worked for a couple of months but the clunk came back. Then the bottom universal joint at the bottom of the steering column was replaced and problem solved. All under warranty.
I had this problem about 18 months ago. The problem was dealt with under warranty and is well known - your dealer is not telling you the truth. At first the joint was lubricated - that lasted for a while but eventually the joint was replaced - problem solved. Try another dealer.