Select all / Select none buttons not working on checkboxes

I know this seems like a tired old question, but despite hours on this and reading previous posts, still no luck. Individual countries tick and highlight nicely, but I can't get the select all/untick all buttons to do anything. Anyone any ideas? http://ift.tt/2hijuOm

<input type="button" id="CountrySelectAll_ID_ws" value ='Tick all' />
<input type="button" id="CountrySelectNone_ID_ws" value ='UNtick' />

<div class="multiselect">
<br>
<label><input type="checkbox" name="option[]" value="1" />Germany</label>
<label><input type="checkbox" name="option[]" value="2" />France</label>
<label><input type="checkbox" name="option[]" value="7" />Spain</label>
</div>




jQuery.fn.multiselect = function() { 
$(this).each(function() {
    var checkboxes = $(this).find("input:checkbox");
    checkboxes.each(function() {
        var checkbox = $(this);
        // Highlight pre-selected checkboxes
        if (checkbox.attr("checked"))
            checkbox.parent().addClass("col-on");

        // Highlight checkboxes that the user selects
        checkbox.click(function() {
            if (checkbox.attr("checked"))
                checkbox.parent().addClass("col-on");
            else
                checkbox.parent().removeClass("col-on");
            alert$("option:selected");
        });
    });
});
};


$(function() {
 $(".multiselect").multiselect();
});


$(function() {
$("#CountrySelectAll_ID_ws").on('click', function()
$('.multiselect').find("input:checkbox").prop('checked',true);
})
});


$(function() {
$("#CountrySelectNone_ID_ws").on('click', function()
{
$('.multiselect').find("input:checkbox").prop('checked',false);
})
});