So I have just received my reply from Merseyrail;
Thank you for contacting us.
I am sorry to learn about the recent difficulties you had with us. Your comments have been forwarded to our Area Revenue Protection Manager. Although you have not told us the date of travel or your interchange station to our network, she has done her best to find out what took place. She assures me that it is not our intention to upset genuine customers such as yourself, but we do need to understand why customers do not have a ticket and she is satisfied that her staff followed the correct procedure.
We do understand that you could not purchase your ticket at your origin station, and, as such, obtained a promise to pay ticket. In such circumstances, customers are asked to purchase their ticket either on the train or at interchange stations if there is sufficient time or if not at your destination. Our understanding is that all Northern Rail services that operate into our stations do have conductors on board who sell tickets. While we do not know your first interchange station, this would only have had a minimum of 5 minutes between trains and we do also understand that if your train was late you may have had time to purchase a ticket at the first opportunity. However, Liverpool Central has 10 minutes for connecting time, and while very much understanding this would have inconvenienced you, we do feel this would have been sufficient time for you to purchase your ticket rather than as you indicate changing directly to the Wirral Line without visiting the booking office or passing through the ticket gates.
Our Area Revenue Protection Manager feels that in the circumstances, the Revenue Protection Officer used their discretion wisely without taking any further action and the advice was given in good faith. Our Officers do only issue tickets for our own network and for all other tickets will direct customers to return to a booking office.
We are very grateful for your feedback about this issue, we don’t wish to upset any customer, and it is important we do comply with the conditions of this ticket and we are consistent in our approach. We will monitor this.
Once again please do accept my apologies for the inconvenience and upset caused, this was not our intention.
So there is a concession that I don't have to delay my Journey at Kirkby however they so seem to indicate that i should exit at Liverpool Central and buy a ticket if I haven't been able to buy one from a Northern Guard.