After all the restarts and failovers, the problem seems to be LA where the passthru (stratum) and the whitelisted node are both having problems still.
That time both lost connection while all other nodes were OK.
When I followed this up with the provider yesterday they claimed they'd tweaked something and the problems also stopped happening, however this return of the problems obviously means it wasn't fixed so I'm awaiting their next reply on the subject.

However, yet again, all is back to normal for the time being.

(FYI "space" and "is" are always on the same servers)