Sadly UUP wasn't able to download the files (tried both the cumulative update and the straight 22000.51) when I ran the compiler :/
Was trying to find a way to fix it, rather than nuke it.
Have now nuked it and it's broken my Insider account :'(
Full nuke (previous was a keep settings) has fixed Insider account, but still stuck on 22000.1 and get crashes trying to update to 22000.51.
Have managed to get UUP working so am building an ISO to do an in-place upgrade.
Out of interest, of everyone that's on Win11, did you go straight from 10 to .51, or go via the .1 version as well?