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:
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.)
I'll add that if they're a legitimate site they'll be mortified but if they're an illegitimate site, they'll quickly move to open up shop somewhere else.
That said, if they're costing you money because of your sentry issues or causing you support issues that you can document, you have just cause for a lot of actions.
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.)