Hi @roadmr thanks for your reply. 
(apologies for delay in commenting; didn’t get notification for this …)
Thank you for your suggestion to create a new app:
fly postgres create --name new-db --vm-size shared-cpu-2x --volume-size 10 --initial-cluster-size 1 --region CHOOSE --org your-org --image-ref registry-1.docker.io/flyio/postgres:13 --stolon --snapshot-id vs_yaddayaddayadda
I re-ran the command to check the snapshots list:
fly volume snapshots list vol_g2yxp4mllj6v63qd
That got me the list:
Snapshots
ID SIZE CREATED AT
vs_vYXP8wx6ezM53CR 96479939 11 hours ago
vs_vnAvN76g36Apwt2A 96479939 1 day ago
vs_azyMXoAexXOG9tA7 96479939 2 days ago
vs_JjO3bbl7nbV2oSX5 96479939 3 days ago
vs_VD9Zm3LRVqQ6lfz7 96479939 4 days ago
vs_gnkGR2jXV7yM3sqN 96479939 5 days ago
Using the latest snapshot: vs_vYXP8wx6ezM53CR executed the suggested command:
fly postgres create --name new-db --vm-size shared-cpu-2x --volume-size 10 --initial-cluster-size 1 --region lhr --org dwyl-auth-546 --image-ref registry-1.docker.io/flyio/postgres:13 --stolon --snapshot-id vs_vYXP8wx6ezM53CR
Creating app...
Setting secrets on app new-db...
Restoring 1 of 1 machines with image registry-1.docker.io/flyio/postgres:13
Waiting for machine to start...
Machine 9080577c1d2798 is created
==> Monitoring health checks
Waiting for 9080577c1d2798 to become healthy (started, 0/3)
Error: context deadline exceeded
It errored twice. 
But for some reason in the Web UI it still shows that it’s running:
https://fly.io/dashboard/dwyl-auth-546
Suggests that the new-db was created (even though the CLI reported “Error: context deadline exceeded”…)
If I attempt to connect to new-db:
fly postgres connect -a new-db
Get the error:
Error: no active leader found
So guessing it’s not working. 
Happy to run any other commands you suggest. 
Thanks again.