Thanks, with that pointer I was able to work out that I needed to do:
sudo ostree pull fedora:fedora/33/x86_64/silverblue --commit-metadata-only --depth=10
(pull commits for last few days)
Then
ostree log fedora:fedora/33/x86_64/silverblue
Which shows me some old commits including this:
commit 20965dea3bda6eb92b30de6a970361d9cf7bea04fe6abb58b405a027b34f3cf5
Parent: 715eda394e11a1c0f357027a885b9ed29efac3942f1846bb912298e898b0642e
ContentChecksum: e47661b41ab3a932053534ad8486d28e16c8ccd07a6d86026c524f1b555490a5
Date: 2021-02-26 23:10:57 +0000
Version: 33.20210226.0
(no subject)
Which I can then roll back to using:
rpm-ostree deploy 20965dea3bda6eb92b30de6a970361d9cf7bea04fe6abb58b405a027b34f3cf5