I finally received a reply from FGW:-
"I was sorry to learn that you were unhappy with our response as you believe the National Rail Conditions of Carriage entitle you to a full immediate refund from any ticket office for an abandoned journey.
I apologise but this is not the case, all refunds must be processed by the original point of purchase as they take the revenue for the transaction. You should be able to submit a refund at London Paddington if the tickets were purchased at another station and they will forward the tickets on to the ticket seller but Paddington would not process the refund itself. Section 26 (a) of the Conditions of Carriage state that an immediate refund will be paid ‘wherever possible’ and unfortunately it is only possible if you paid by cash and submit the refund to the point of purchase."
Discusson on here seems to suggest that 'wherever possible' relates to simple things like, for instance, having sufficient cash to hand to make the refund. East Coast, in their reply to the complaint, stated that both them or FGW should be able to make the refund. Now, who do I believe?