You don't have to pay. If you think that your case is strong enough to defend yourself against possible charges in court, then you try this.
However, asking for a ticket of a distance shorter than the one that you have travelled is usually interpreted as deliberately trying to evade the fare. If it wasn't, ask yourself how the court could ever differentiate between someone who has made a mistake, and someone who has done this deliberately? This allows for prosecution under the Regulation of the Railways Act which would result in a fine and criminal record if successful.
At £80, you can consider this an expensive mistake. You could take the approach of sending in a lower amount - Northern could accept that you made a genuine mistake, and accept the lower payment. But it's a risk and we have no reason to believe that they would accept the lower amount; we know that they would accept the £80.
However, asking for a ticket of a distance shorter than the one that you have travelled is usually interpreted as deliberately trying to evade the fare. If it wasn't, ask yourself how the court could ever differentiate between someone who has made a mistake, and someone who has done this deliberately? This allows for prosecution under the Regulation of the Railways Act which would result in a fine and criminal record if successful.
At £80, you can consider this an expensive mistake. You could take the approach of sending in a lower amount - Northern could accept that you made a genuine mistake, and accept the lower payment. But it's a risk and we have no reason to believe that they would accept the lower amount; we know that they would accept the £80.
) I am being dim.