I have another reply from customer service after my follow up email on 5th march.
below is an extract which explains why i and others are not allowed an excess on trains from certain stations. any comments?
"I can confirm that the National Rail Conditions of Carriage do make provision for a Train Company to allow for upgrades or Excess Fares on the train. However, we only allow for this at those stations without ticketing facilities.
Part of the reason for this is that ticket checks/sales only form one small part of the overall responsibilities of our Train Managers - their primary responsibility being to the operational running of the entire train they've been assigned to. This includes overseeing dispatch duties, attending to safety-critical procedures, managing performance demands and offering mobility assistance etc.
Our Train Managers are only provided with the hand-held ticketing machines only to offer ticketing services for those customers boarding at unmanned stations - as a supplementary measure, rather than representing an outright alternative to station facilities. So, if the station a customer boards at has dedicated Booking Office facilities, we do not see it as unreasonable to expect the passenger to use them. That way, it eases the demands on our train crew, and leaves them free to attend to their primary duties. Indeed, the Clerks at the station Booking Office are specialists in rail industry retailing practices, and so really are best-placed to help with changing any tickets (whereas retailing issues only form a lesser part of the Train Manager's overall training/duties).
As per our email of 27 February - and the stipulations of our Buy Before you Board policy - if the station a passenger boards from has a manned Booking Office, then the customer is expected to buy any upgrade or Excess before boarding a train. This is consistent with railway bye-law, and means our crew should not be issuing Excess Fares on-board the train for customer boarding from a station with a manned Booking Office. I appreciate that this will not always be as convenient as some customers would hope for, and I am truly sorry if this comes as a disappointment.
I appreciate that you may interpret the National Conditions of Carriage differently. However, the key phrase here would be that of "...this rule does not apply in designated Penalty Fares areas..." Essentially, the Penalty Fare areas are those at stations with manned Booking Offices. So again, the rule is that if the station has a staffed Booking Office, then the customer is expected to make use of this to make any ticket changes prior to boarding. I'm sorry if our original email wasn't clear on this.
Given the way Excess Fares are calculated (the fare being dependent on the original ticket held) I regret that they cannot be issued by the automated ticket vending machines. Similarly, Excess Fares cannot be arranged via our website, simply due to the inherent limitations of such automated sales facilities. So, as much as I recognise and appreciate the frustration of having to queue at a Booking Office, the customer would have to do so in order to change their ticket."