The more professional you can make it look the better chance you won't have to resort to a solicitor.
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.)
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.