I agree with this approach. You don't need a lawyer for most things of this nature. They're expensive and finding a good one can be a real crapshoot.

I don't think there's anything wrong with researching the legal side of things a little bit, then firing off a letter that politely but firmly disagrees with their position.

If they're really serious about coming after you, you'll find out soon enough, and you can get a lawyer then.