Apart from the subjective judgements which a court must make, to sustain a criminal conviction for harassment the behaviour must consist of “a continued course of conduct involving a number of events carried out by the same perpetrator(s) on the same victim”. A “number of events” has been determined to be two as a minimum, but usually to secure a conviction more than two would be needed. You will note from Eddie’s link “What counts as harassment” that most of the examples quoted meet that definition. They describe ongoing courses of action. A single threat of violence is not harassment but it would be if accompanied by other threats, intimidation or embarassment.
Simply stating once that “Mr NJ owes me £1,000 and went to court to get the sum reduced or written off” is not harassment. If it’s true nothing can be done, if it’s untrue it becomes a matter of slander (if spoken) or libel (if written) for the civil court.