From the
'Advance Purchase' Terms & Conditions on National Rail Enquiries:
If the train you purchased a ticket for is cancelled or delayed by more than 60 minutes, special arrangements will be made to accommodate you on another train (although a seat cannot be guaranteed). If, as a result, you decide not to travel, a refund will be offered on completely unused tickets and you will not be charged an administration fee.
Therefore, if your train is cancelled you are entitled to a full refund. Send your tickets to the address below (best to keep photocopies/scans of them) with a covering letter explaining you are requesting a refund as per the conditions on National Rail Enquiries.
FGW▸ Refunds
PO Box 23972
Edinburgh
EH3 5DA
I have checked
NRE‡ and I can see that there is no 1030 service from Manchester Piccadilly to South Wales on Sunday 20th, other services that morning are shown as diverted between Crewe and Shrewsbury - can't see info about replacement buses - perhaps the details have changed again since you enquired at a station. WebTIS based booking engines (redspottedhanky.com, East Coast) also do not show a 1030 service. But trainline.com based booking engines (FGW) are still showing the 1030 and taking bookings. Suggest in your request for refund you make it clear that the information provided by National Rail Enquiries (the definitive source) shows that your booked train is no longer running at a time convenient to you. I suspect the 0930 is your train, leaving an hour earlier because of the diversion. That to me still counts as a cancellation and eligible for a refund.