I'd guess the sum included interest but on a more serious note, if it was a genuine debt that we owed, why wasn't it paid?
Our government say that long-bearded idiot came into power in Iran (won't name him for fear of death sentence) and they were worried about the money going into terrorist hands, but not much has changed besides the beard getting shorter. By Hunt's own admission, we owed them the money, so it appears the only way Iran managed to get money duly owed back was by kidnapping an innocent woman and holding her hostage for 6 years.
If Nazanin Zaghari-Ratcliffe’s release was due to the £400M debt being paid you have to question why it wasn't paid 6 years ago, if not 45 years ago. Somebody's got questions to answer.