Up to 50 downloads and zero profit for you, minuscule to zero exposure to Amiga fans (zero to general public!).... I just don't see how they could prove any loss of earnings.... I have difficulty taking this seriously because of that. It's their job to do this but in your case there has been no loss to Studio Payo and so no claim can be made.
Make it clear that further correspondence from yourself will be charged at £10,000 per letter.
As for the €500 crap, I hope you're not thinking of paying it? And besides, emails don't count in this case because there's no guarantee you'd check it or it wouldn't end up in spam or something.