I have been persistently contacted and pressured by AMEX for almost 1 year after paying the debt off to their debt collector in full. I have contacted AMEX on multiple occasions telling then this but they say it's still outstanding. Insultingly the have now offered to reduce the debt. The payment has gone to their nominated debt collector and I've double checked that.
The reason I want to think about legal action against them is for the psychological harm their debt collection practice caused to me. I don't want to go into details here but my doc is satisfied it was down to that.