Thank you. So stick this in the theme functions, set to the correct form ID and add the same args per my wp_query. So I have ended up with:

function filter_function_name( $query_args, $sfid ) {
    
    //if search form ID = 225, the do something with this query
    if($sfid == 4672)
    {
        //modify $query_args here before returning it
        $query_args['numberposts'] = -1;
        $query_args['posts_per_page'] = 25;
        $query_args['post_type'] = 'product';
        $query_args['post_status'] = 'publish';
        $query_args['meta_key'] = 'special_offer';
        $query_args['meta_value'] = true;
    }
    
    return $query_args;
}
add_filter( 'sf_edit_query_args', 'filter_function_name', 20, 2 );

I have left the rest of the settings as per my last message (including the wp_query on the special_offers template where I am displaying the products). When I view the page the filters are not populating as expected, I should be seeing options for colour and size but I just see one colour option, which when I apply renders no results.

Am I doing something else wrong here?

Sorry for the late reply. OK so I already have those query arguments in the function filter which is in functions.php. Now if I remove them from wp_query the page just has a standard WordPress loop to display content right? I have set the results method to “Custom” and defined the correct Results URL in Template Options – and yes I have the form set to display products & variations. With this setup I get no results at all. I can supply screengrabs of any sections that may help?