The correct way of looking at this, though doubtless it is not a "happy way" is that the real cost of this to you is £100 - the admin fee being charged by the TOC plus any "lost fares" - ie any fares you have paid already which have been disregarded by the TOC in their calculation of the fares you should have paid.
I would pay the sum to settle the matter but then I think I would seriously consider making a complaint to Trading Standards as to how the booking system displays fares and any restrictions which may apply to the use of railcards in purchasing those fares.
Some of the wording used by TOCS is less than clear and arguably ambiguous. After you have settled I would ask the TOC if they hold the Plain English Campaign Crystal mark and if not why not.
I would pay the sum to settle the matter but then I think I would seriously consider making a complaint to Trading Standards as to how the booking system displays fares and any restrictions which may apply to the use of railcards in purchasing those fares.
Some of the wording used by TOCS is less than clear and arguably ambiguous. After you have settled I would ask the TOC if they hold the Plain English Campaign Crystal mark and if not why not.