OK so resolved this and the other issues affecting the upgrade and successful implementation of PS1.7. Unfortunately, having managed to get everything over to 1.7 I have to say, despite the efforts of everyone involved in the open source program, there is nothing particularly worthwhile in upgrading apart from (possibly) some better managed coding and security whilst the backend remains pretty much the same and the frontend is far less flexible or even pleasing to the eye. Having said that, and to possibly assist anyone else attempting to upgrade, I've listed all the steps below that I had to take to successfully upgrade our 1.6.1.24 shop to 1.7, and whilst I can't be sure if it will help anyone, here's the info anyway ....
Reverted to PHP 7.1 from 7.3 (according to the docs 7.3 "could" be supported, but I found issues when trying to upgrade on PHP7.3)
Backup home directory & dbase
enable US-EN & set default (remarkably, it appears that if you DON'T have us-en enabled and as default, the upgrade will either fail or cause issues). Additionally, it also appeared that by leaving our native language enabled as default, thus affected our ability to edit products with attributes after the upgrade, something which has affected many upgrades, the answer for us was to simply make sure en-us was default and our native language was left enabled, after the upgrade we disabled the en-us and made our native language default again, now able to edit products no problem.
Enter Maintenance Mode
Update 1-click-update to 4.10.1 (or latest) - even though we didn't receive any notifications via the dashboard of an upgrade available for 1-click-update, THERE WAS AN UPDATE AVAILABLE.
In Advanced => Performance
Disable non-native mods
Disabled overrides (we raised an issue as a bug, namely that users couldn't log in and new users couldn't register after the upgrade and were told to check we weren't using "overrides". It turns out that even disabling the overrides didn't make a difference and they REMAINED ACTIVE DESPITE DISABLING THEM so it's worth remembering you MAY still have active overrides even though you disable this from the back end, how you figure that out and deal with them is anyone's guess but more on our issues below).
Clear all caches
Run 1-click-update
After Upgrade and managing to log in to back office
=============
As a precaution, clear all browser cache, particularly if you are installing/testing on the same domain.
Delete contents /var/cache/*
Delete table ps_access and restore using the attached ps_access file earlier in this thread (this resolved the "access denied" message most people see after the upgrade).
OVERRIDES (as mentioned earlier, unknown to us we had overrides on the site and couldn't disable them despite having done this from the back end, apparently it's a bug and for us the issue was users not able to log in or register after upgrade. We traced it to the following:-
Edited override/classes/customer.php (must have been manual override)
FROM:
'passwd' => array('type' => self::TYPE_STRING, 'validate' => 'isPasswd', 'required' => true, 'size' => 32)
TO:
'passwd' => array('type' => self::TYPE_STRING, 'validate' => 'isPasswd', 'required' => true, 'size' => 60)
Updated ALL modules
In Localization
Change your language back to whatever your native language was and disable en-us if not using it.
A multitude of front end issues for us, namely alignments of menus, etc that all looked pretty ugly, along with the main menu just randomly throwing sub-cats all over the place with no thought to any type of alignment for the drop down menus, etc, tried installing another menu mod but that was just as ugly. Eventually just gave up and went with the default menu.
However, on the product page, we noticed the descriptions aligned to the right of the page, rather than full width, here's how to sort that (not my code, taken from the forums)
How to set full width product description and features on product page?
=======================================================================
Open: /themes/classic/templates/catalog/product.tpl
Find line containg:
{block name='product_tabs'}
Insert code before this line:
</div>
</div>
</div>
<div class="row">
<div class="col-md-12">
<div>
Additionally, there is a huge waste of space on category headings, where presumably a picture is meant to be placed, but in our case we didn't want that, and we were happy to just remove that space. Here's how:
Remove big space from category header
=====================================
Remove following code from /themes/classic/templates/catalog/listing/category.tpl
{block name='product_list_header'}
{include file='catalog/_partials/category-header.tpl' listing=$listing category=$category}
{/block}
Remember, when editing the templates you may need to flush the cache from the back end.
So all in all, a few weeks of hard slog to simply get to the point of the upgrade working, but with the front end not particularly pleasing to the eye. We've decided to stay with 1.6 for the time being, and are currently looking at migrating to WooCommerce where it appears there is a great deal more support and they appear to be leading the way for ecommerce now and even having already tried a free module to migrate prestashop to WooCommerce worked flawlessly which raises the question of why was it so difficult to update and migrate to another version of prestashop when it is so easy to migrate to a separate e-commerce system?. I understand it's pulling the detail from the dBASE and not upgrading the files, etc, therefore we may suggest that it's a simple migration procedure of data but we've never really had an issue with updating the files as such, it's always been an issue with the dBASE but I can't profess to know precisely what's involved.
For us, it's a sad situation because we've loved using Prestashop and still do, but the thought of "putting up with" 1.7 doesn't appeal for now, we may change our minds, I'm not sure, but either way, I hope some people find these instructions useful.
All best
Marko