Using hikashopModifyQuantity

// function toggles seat image and alters quantity in basket
function toggleSeat(seatID,productID,category)
  {
   var x=document.getElementById(seatID).innerHTML;
   var product; // final product ID (corresponding to adult or concession)
   var y;
   var field = new jmt_field;
   
    if (x.search("seat-category") > 0) {
        y=getRadioCheckedValue("ticket_type_form","ticket_type");
        if (y == "Concession") {
            document.getElementById(seatID).innerHTML="<img src=\"http://myco.org.uk/seat-chosen-concession.gif\" title="+seatID+">";
            product = productID + 2;
        }
        else {
            document.getElementById(seatID).innerHTML="<img src=\"http://myco.org.uk/seat-chosen.gif\" title="+seatID+">";
            product = productID + 1;
        } 
        
	field.value = 1;
    }
    else {
    	if (x.search("concession") > 0) {
    		product = productID + 2;
    	}
    	else {
    		product = productID + 1;
    	}
    	document.getElementById(seatID).innerHTML="<img src=\"http://myco.org.uk/seat-category"+category+".gif\" title="+seatID+">";

		field.value = 0;
    }
    
    if (hikashopCheckChangeForm('item','hikashop_product_form'))
    { 
        return hikashopModifyQuantity(product,field,1,0,'cart');
    } 
    else { 
    	return false;
    }
}