I would reply in a letter - not on the form - saying.
"Thank you for your recent letter, which I attach.
On the date I travelled, I was travelling with an Off Peak ticket (give full details of the journey) and your representative at Redhill noticed this was not valid at the time I was travelling. They then charged me the excess fare to correct the difference, the transaction for which I attach to this letter.
I trust that having paid the correct fare this closes the matter to everyone's satisfaction.
Yours faithfully"
I don't think this was the case. The OP realised it was the wrong ticket and actively sought out the inspector at Redhill. Further, the gateman at Coulsden South let the OP through the barrier effectively saying the ticket was ok.