I'm interested, as I'm sure others are, as to how you define your product. By saying that, you're implying you owned the complete rights to those exhausts, and one would have assumed that you had a production agreement with JP exhausts so that they could produce these only for you (let's be honest, a very basic bit of business). Therefore, if JP had wanted to produce the exhaust for anyone else, they wouldn't have been able to, unless they'd obtained your permission in writing.
You are correct, JP don't see if that way though!
Alternatively, you didn't/don't actually own the rights to your exhaust, but instead just sold JP exhausts under the "Yozza" brand, and you had some sort of a hand in the design.
If that was the case then JP would've sold direct to the public and other trade, yet they sent all enquiries to us for 7 years!
If it is the first, then I doubt this discussion would be taking place instead on a Clio forum / in public, as it most probably would have been sorted out by lawyers, and either JP would not be allowed to sell to anyone else, or they would be paying you a royalty for every one sold.
throwing £10K at lawyers isn't really going to happen, the damage is done!
If it is the second, then actually it wasn't your product, and you've just learned a valuable lesson in business, so quit whinging about it!