Problems installing PiTiVi via Flatpak

[Bob Thomson]

I have spent a huge amount of time trying to install PiTiVi using the 
instructions at
https://flatpak.org/setup/Ubuntu/

then https://flathub.org/repo/appstream/org.pitivi.Pitivi.flatpakref

I think this worked.

But it keeps failing to download files as you can see from the terminal 
output below

~$ flatpak install flathub org.pitivi.Pitivi
Installing in system:
org.pitivi.Pitivi/i386/stable
flathub 5439378d3e5d
   permissions: ipc, pulseaudio, wayland, x11, dri
   file access: host, xdg-run/dconf, ~/.config/dconf:ro
   dbus access: ca.desrt.dconf, org.freedesktop.Notifications
org.pitivi.Pitivi.Codecs/i386/stable flathub a613c7a70e99
org.pitivi.Pitivi.Locale/i386/stable flathub 43ab493d0725
Is this ok [y/n]: y
Installing: org.pitivi.Pitivi/i386/stable from flathub
[####################] 272 metadata, 2720 content objects fetched; 45282 
KiB transferred in 353 seconds
Now at 5439378d3e5d.
Installing: org.pitivi.Pitivi.Codecs/i386/stable from flathub
[####################] 5 metadata, 23 content objects fetched; 8604 KiB 
transferred in 21 seconds
Now at a613c7a70e99.
Installing: org.pitivi.Pitivi.Locale/i386/stable from flathub
[-                   ] Downloading metadata: 1/(estimating) 569 bytes 
(569 bytes/s)
Warning: Failed to install org.pitivi.Pitivi.Locale/i386/stable: While 
pulling runtime/org.pitivi.Pitivi.Locale/i386/stable from remote 
flathub: Server returned status 503: Service Unavailable