$_GET data from Search Form getting lost

I’ve inherited a WP site, the previous webmaster has moved on.
There are several plugins, (see list below)
Client asked for a Search, so I loaded several standard plugins, but they all fail.

On examination it appears the contents of $_GET and $_POST which should contain the Search input field have been deleted.
I have tracked this back to the start of wp-settings.php (see diagnostic below), but I have no idea where to go next.

It looks like the input data is being removed by WP before any plugin is called, but this seems somewhat unlikely to me.

Any suggestions where to look next?
eg likely places to drop diagnostics to see where the input data is being lost.

Thanks
Malcolm

1, Plugins
jNewsticker for WordPress
Auto Image Resizer Pro
Email Login
GDHL Custom User Contact Fields
Gravity Forms
Gravity Forms User Registration Add-On
WP-PageNavi
XML Sitemaps

2, Diagnostics at start of wp-settings
define( ‘WPINC’, ‘wp-includes’ );
echo “AAAAAAAA wp-settings
“;
echo “AAAAAAAA _GET “;print_r($_GET ); echo “BBBBBBBBB
“;
echo “AAAAAAAA _POST “;print_r($_POST ); echo “BBBBBBBBB
“;

Entering “QQQQQQQQQ” in the search box produces
AAAAAA wp-settings
AAAAAAAA _GET Array ( ) BBBBBBBBB
AAAAAAAA _POST Array ( ) BBBBBBBBB