Looks like there will be Seat Reservation Fees on some National Express routes.
The fee does not apply to train tickets bought and paid for in advance. Given the savings to be made by buying tickets in advance this probably means that the majority of passengers will not have to pay reservation fees.
The reason they give is that, with the free reservation service, people were reserving seats on several trains for the same journey. Thus causing not a little inconvenience to other passengers who had not reserved seats themselves and could not be sure whether any empty seats would be used or not.