Time to restore from a backup or OS-copy from another server if necessary. Of course for names.nsf you’ll have to bring down the server to make the switch to the good file.
My original reply seems to have been deleted, but as it happens this wasn’t a problem with the db on the server, it was my client’s bookmarks that had obviously got corrupted in some way.
After removing bookmarks I could open the names.nsf (and I noticed another couple of dbs with the same problem ,but not many)
Rather strange and I think it must have something to do with the fact I’d just converted myself to roaming (which is another story)