I agree with Jim, go on the front foot, start with a strongly worded letter to the individual who last sent you a correspondence followed by a statement of claim for damages and go big, tens of thousands. If they fail to answer your statement of claim in full and to your completed satisfaction. Send another reminder of it with the amount you demand. If they don't answer that then send them notice of default by which they have legally accepted your claim and you win in any case brought to court, do it by registered mail so you have evidence.
Pp