Dell Venue 8 5830 - update (W10) leaves no input box on (blurred) lock screen

I have a Dell Venue 8 5830 tablet, which I've been using with Windows 10 for some time.  I've just loaded the latest updates, including a "Cumulative Update".  Now, when I try to log on, the lock screen blurs (by design, apparently) but the usual input box for password/PIN doesn't appear.  A web search suggests a common solution is to re-set the service "Credentials Manager" to automatic, but I can't get anywhere near this.  I've tried connecting an external keyboard via an adapter, but the first character of Ctrl-Alt-Del simply blurs the lock screen as before (repetition doesn't help).

If I turn off Secure Boot I can use the Vol-down button to get "boot options", but then I get a dialogue about recovering Bitlocker keys.  If I skip that I can get into options to reset (simply does nothing) or I can bring up a command prompt.  If I could figure out a way to set Credentials Manager to Automatic that way I might just be able to fix it?

Looks like a Windows Update has bricked this nice little tablet.  Any ideas?