Many thanks to those who have replied to my query. I did phone Passenger Assist at Great Western Railway and Cross Country, the belt and braces approach, to ask about excessing. Both told me to do it on the train via the train manager, should the Disabled Railcard not turn up.
That really is unhelpful. There is not supposed to be a way of excessing a fare to remove a railcard discount, the conditions of carriage are quite clear that if no railcard is carried then a new ticket is to be sold, and a refund given afterwards (limited to only once per year) if entitlement can be proved retrospectively.
As is often the case
TOC▸ helplines, Twitter reps etc etc are often worse than useless about detailed fares rules.
Paul