To others, here's what fixed it for me.
1. Stay Calm.Remember you have a backup as a last resort (you DO have a backup right ?)
2. Upload Rob's script (not the zip file, just json-db-check.php) to your site's public_html directory.
3. In a browser go to www.yoursite.com/json-db-check.php
4. It will auto-fix a load of db errors for you.
5. If your site is still broken, click the blue button to do a full check.
6. Now you'll see a list of tables, one or more may have 'not valid JSON' code reported, our site had this:
7. Via cPanel use phpAdmin (or find someone confident to do so). Our error was in line 11 (com_languages) so we clicked Edit on that line.Checking table: xyz_extensions, column params
Row 11 is not valid JSON. Error: (Syntax error)
Content: {"administrator":"\"en-GB\"","site":"\"en-GB\""}
8. Once it's open for editing, scroll down to the params field. Note exactly what is there so you can add it back in if required.
9. There is advice here about copy / pasting whatever is in params to a JSON validator like jsonlint.com. Our code returned as valid. <shrugs> So we just removed everything except the { } open & close braces.
10. Scroll down and press Go.
That resolved our issues, both site & admin now ok.
