Replies to the above post made it clear that GWR Advance Singles were not refundable after March 2020 - which was a pain because I bought my £70s worth in SeptemberI bought a GWR Advanced ticket on 5/9 for travel 28/9 - which I was then not able to use due to local lockdown.
Acting on a suggestion above, I have used the WayBackMachine to view GWRs cancellation policy at the time of booking.
There are only 3 caches - one before pandemic and 2 recent - as such no mention of allowing refunds on Advanced . . . . BUT
On 14 Aug, their website stated "On 7 September we will be reverting to our normal refund policy" - does this mean Advance Tickets were refundable up to 7 Sept? So my ticket bought on 5th Sept?
I am pleased to report that I have today received a full refund through the booking site TrainPal, having requested a refund and citing the fact that the contract had been 'fustrated' as a result of lockdown.
I don't know if I was lucky the tickets had been purchased through a 3rd party agent, or if GWR would react in the same way, but others seeking refunds might want to try the 'fustration' argument
Definition of frustration in the Legal Dictionary by The Free Dictionary
In the law of contracts, the destruction of the value of the performance that has been bargained for by the promisor as a result of a supervening event.