HP ENVY 6400 Series Scanner Driver Plugin Install How To

Successfully installed hplip-3.21.4 on Xubuntu 16.04 and 18.04 (see Question #709372). Printing good, now attempting to run scanner using Xsane v0.999 on desktop PC Xubuntu 16.04.
Selected HP6430e from Xsane entry dialogue and get msgs:
    Failed to open device 'hpaio:/net/ENVY_6400_series?ip=192.168.1.203':Error during device I/O
    Driver Plug-in Required
        HP Device proprietary plug-in is missing. Click 'Next' to continue plug-in installation. In case of plug-in download failure,
        run 'hp-plug-in' command from terminal manually.
Click 'Next, and immediately got a further error msg - there was no opportunity to enter a password:
    Failed to install Plug-in.
    Either you have chosen to skip the Plug-in installation or entered incorrect Password.
Opened terminal and ran hp-plug-in:

jg@jg-Inspiron530:~$ hp-plug-in
No command 'hp-plug-in' found, did you mean:
 Command 'hp-plugin' from package 'hplip' (main)
hp-plug-in: command not found
jg@jg-Inspiron530:~$

So ran hp-plugin.
In GUI dialog selected 'Download and install the plug-in from an HP authorized server (recommended)' and clicked 'Next'
After a long wait (50secs) with substantial CPU activity got error msg:
    file does not match its checksum. File may have been corrupted or altered
I suspect that the file request timed out and there was nothing to check against the checksum. The terminal output was as follows:

jg@jg-Inspiron530:~$ hp-plugin

HP Linux Imaging and Printing System (ver. 3.21.4)
Plugin Download and Install Utility ver. 2.1

Copyright (c) 2001-18 HP Development Company, LP
This software comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to distribute it
under certain conditions. See COPYING file for more details.

HP Linux Imaging and Printing System (ver. 3.21.4)
Plugin Download and Install Utility ver. 2.1

Copyright (c) 2001-18 HP Development Company, LP
This software comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to distribute it
under certain conditions. See COPYING file for more details.

Checking for network connection...
Downloading plug-in from:

error: Found No Section in /tmp/tmpNFvBaW. Please set the http proxy for root and try again.
error: file does not match its checksum. File may have been corrupted or altered

Done.
jg@jg-Inspiron530:~$

Repeated the exercise on a core8 laptop running Xubuntu 18.04 and hplip-3.23.12 with exactly similar reults - the final wait for the download error took 30 secs.

1. What do I need to do to get the plugin?
2. Is it possible to get the necessary plugin and install manually, and if so, how?
Any clues would me much appreciated
Tks
jg