WSUS corrupted: Can’t uninstall, repair, or remove (Error 0x800f0831)

Have a single server site, running Windows 2012 R2

They had a long power cut, and the server uncleanly shut down when the UPS gave out.

We had a few other programs and database issues, but I've worked through them all, apart from Windows Server Update Service:

The WSUS MMC would not open, and just give an error (This has been going on for a few months, and the original message has been lost in the mists of time, but it was not the Manual steps needed for KB3159706).

WSUS was not installed using the Windows Internal Database: it's a separate 2012 R2 Express instance.

I've tried various fixes, including repairing the database, but couldn't get anything working, so I thought I'd try removing the WSUS Service and database roles.

In server Manager, I can't Add the WSUS Services and Datbase roles, as (reasonably) it's already installed.

In server manager, if I try to remove the Database role, I get this message:

The Request to Add or Remove features on the specified server failed. an unexpected event has occurred. You can view event logs in Event Viewer to learn more about possible causes for this problem. Error: 0x800f0831

Searching through event viewer (mainly the system and application logs), I can't track down any relevant events that seem to tie into this message.

Only bit I have managed to remove is the SQL Express instance using the SQL Add/remove programs feature.

Doing some googling on the error code, Windows Role adding and removal, and WSUS issues, I can't seem to find any info that applies.

Can anyone assist? In my mind, I'm hoping there's a procedure that basically says "Uninstall the role, ignoring any errors" (Or "Re-install/repair the roles over the top") which will let me add the roles from scratch – It's a very simple WSUS setup, so happy to just nuke the configuration, and start from scratch, if there's a way to do it.