As much as many people find watermarks distasteful, this is another example where they might have made them think twice about using them.
Perhaps you should pose yourself as a potential client to the design company so that you can at least get to talk to a real person. In some cases it is an employee that did this without knowing the full regulations. I had this with a series of shots of mine once and while it took awhile to find the right person to talk to, they came forward and offered me compensation and a letter of apology. They say they fired the person that apparently lifted my images but who knows for sure.
Be persistent. If you do finally get to talk to someone, don't go in with both barrels blazing. Position it in a way that they have done something immoral, if not illegal, and you are willing to work with them to find a mutually satisfying agreement. If you go in and threaten lawsuit right away, you will probably get little, if anything after you have finished paying a lawyer.
I'm not acquainted with your regulations in the UK so I can't help you with the legality side of things.
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