Is erg-services a category?
If so we might be able to do this:
add_filter( 'generate_logo_href','generate_add_custom_logo_href' );
function generate_add_custom_logo_href()
{
if ( in_category( 'erg-services' ) ) {
return 'http://test.ergapc.co.uk/erg-services/';
}
return 'http://test.ergapc.co.uk/';
}