So long as you are keeping hold of the amp until payment is received in full I can't see what he can do to you.
Logically if he's a player he can't realistically take his from Poland and if he's doing skilled labour like building or plumbing work he is likely to have some disposable income (although my understanding of eastern European immigrant workers is that they tend to use their time in Britain to learn skills and build up savings so that they can return to their respective countries and possibly retire or build themselves a nice house. Quite frankly I'd say they are deserving of a good retirement as they often have a very strong work ethic.).
That previous paragraph is so full of stereotype and generalizations, I apologize.
