Personal debt or something contracted?
Personal debts aren’t easily enforceable, you’ve got to prove they are exist for a start.
Truthfully it’s not easy, you’d need to do proper leg work like ‘stake out’ relatives addresses etc and if it all goes wrong it’ll be you in trouble with old bill.
If it was me… hypotheticaly the easiest way to track someone is find someone wio has access to credit reference searches – they hold records of address links, or maybe open try to open an account in the miscreant’s name with Noddle or Experian and search for addresss links, I wouldn’t really though as it would be illegal.