Ah I also get the following error...
Error org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.NetworkManager was not provided by any .service files
Is it necessary to stop the network service before suspending?
I've now changed the script to
dbus-send --system --print-reply --dest=org.freedesktop.UPower /org/freedesktop/UPower org.freedesktop.UPower.Suspend
That does work, but I'm left with the doubt about the network.