You rewrite rule is quite broad and will most likely generate a lot of conflicts.

    add_action('init', 'dcc_rewrite_tags');
    function dcc_rewrite_tags() {
        add_rewrite_tag('%propref%', '([^&]+)');
    }

    add_action('init', 'dcc_rewrite_rules');
    function dcc_rewrite_rules() {
        add_rewrite_rule('^cottage-details/(.+)/?$','index.php?page_id=2&propref=$matches[1]','top');
    }

Then you can access to `propref` query var like:

    $propref = get_query_var( 'propref' );

And remember to flush the rewrite rules; you can do it by going to Settings -> Permalinks and clicking on save button.

**Note**: changed `p` query var to `page_id` because, as you said in the comments, you are using a page, not a standard post.