I've found the solution and I thought it worth adding here because it's something most folk who just basically maintain and pop a couple of mods in ZC once in a while with lots of crib sheets (like me) may not think of and there is very few (I found just one hidden away) references to the problem.
I was receiving 1146 and 1054 database error codes and there is pretty much nothing in lay-mans terms to deal with these outside of an upgrade of the website so it's hard to dig out the cause.
The errors actually tell you what is wrong but unless you understand what they are saying, and why, it's hard to work it out. It was basically telling me the tables didn't exist, I checked and they did exist so it seemed nonsensical until I unearthed a small post from someone with the same problem.
If you get these error codes for mods outside of an upgrade, check your configure files in admin/includes/configure.php and includes/configure.php for your database prefix, if you use a prefix like XX_ that could be the cause of the database errors. Also check that the database tables to which the error codes refer do exist.
I use a table prefix for the database, it's not uncommon especially if you create a database from an install programme. The install.sql file for the mod does not include a database prefix for the tables in this file. That was why the errors were telling me the tables didn't exist, the names didn't match. Duh!
So if your Google Merchant Centre mod returns 0 out of X products processed, here's what to do.
Back up your database and go into PHPMy Admin. Check the relevant table is there and if you have a database prefix, in my case it is something like XX_configuration and XX_configuration group. Open up the install.sql in your favourite text editor and change the references to these tables in your install.sql to include the prefix but don't do as I did and change all the fieldsets as well (like configuration_group_id) cos it won't work and the 1054 error message appears
Copy and Paste the changed text into the SQL field on PHPMy Admin and press go.
Now if you're like me and upgrading this module rather than creating an initial install you might find the following error (as I did)
1062 Duplicate entry 'GOOGLE_FROOGLE_USERNAME' for key 2
Back up the database (standard practise, you screw up and you lose it otherwise, there's no sympathy to be had if you do, just lots of sniggering or tutting from the more po faced)
Go into the XX_configuration table and sort by the configuration_key (basically just click the link).
Find all the names that begin GOOGLE_BASE or GOOGLE_FROOGLE and delete them.
***WARNING*** Don't do this is you are a numpty, easily distracted or even slightly unsure .... Step away from the computer now!
Ask someone with a clue to do it if in any doubt, otherwise if you delete the wrong bits you'll just end up losing valuable parts of your database and will have to install you back up and start over again. The time and trouble is worth a couple of tenners or so to get it done properly.
The copy and paste your install.sql into the SQL box and press go.
Your Google Merchant Centre mod should now be working in your website admin.
If it isn't ... sorry I haven't a clue
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