You need to get `a proffesional` such as a solicitor, company director, teacher etc, preferably a solicitor to buy one, say you need it recorded delivery, before it gets delivered you need someone to witness your work that you have (if not already done) and then post it to yourself
Then work out a claim and sue them in court for your loss, they will not attend (99% chance) and you will win without a fight, if you have your evidence then you will win anyway.
Then , If they have sales, send them a letter (preferably witnessed by someone like solicitor) claiming a royalty fee for each copy sold, claim no individual royalty exists and make a whopping great charge for the year, per year, minimum number of years.... , they will not pay,add interest, letter fees, fees for collection, solicitor costs, you can then sue them in high court and apply for there bancruptcy.