Thanks @michalc !
There have been no changes to the network that should affect the upgrade. I had upgraded the organizational level MR firmware just prior, however no upgrades were actually performed due to those devices no longer supported on current firmware - it was more just to clear the alert. I had also restarted our spare MX that morning. The main uplink was always online.
The only switch that rebooted itself was the MS225 which upgraded successfully.
Initially I tried a staged upgrade, with the first stage being a single MS220. That hadn't completed after 40 minutes, still saying "Started - not running configured version", so I called your support team. We manually rebooted, but no change. We then rolled back, and tried a "perform now" upgrade of all switches, which is when only the MS225 worked. We tried rebooting 2 of the MS220, but no change. Nearly 2 days later now, all 4 x MS220 still say "Up to date - Not running configured version".
I'm expecting an update from your support team on my case today. Thanks