You don't have to necessarily resort to a solicitor/lawyer in this scenario. If they are using your exact HTML/CSS/JavaScript and don't quickly respond and/or comply, you can always file a DMCA takedown notice:
http://www.smashingmagazine.com/2009/12/18/my-website-design...
Most hosts will quickly take down the website, giving you a cost-effective way to stop the issue. However I'd strongly encourage you to pursue a more friendly approach first, to give them a chance to do the right thing. (The site owner may not be the one who actually stole the design.)