Magento observer add column to sales/order grid

config.xml

<?xml version="1.0"?>
<config>

    <adminhtml>

        <events>
            <core_block_abstract_to_html_before>
                <observers>
                    <sales_order_grid_transaction_state>
                        <type>model</type>
                        <class>aardvark_mods/observer</class>
                        <method>updateSagePageSalesOrderGridColumn</method>
                    </sales_order_grid_transaction_state>
                </observers>
            </core_block_abstract_to_html_before>
        </events>
    </adminhtml>

</config>

Observer.php

<?php

class Aardvark_Modifications_Model_Observer
{
   
    /**
     * @see Ebizmarts_SagePaySuite_Model_Observer_Sales::addColumnToSalesOrderGrid()
     *
     * @param $observer
     * @return mixed
     */
    public function updateSagePageSalesOrderGridColumn($observer)
    {
        $block = $observer->getEvent()->getBlock();
        //if (get_class($block) == 'Mage_Adminhtml_Block_Sales_Order_Grid') {
        if($block instanceof Mage_Adminhtml_Block_Sales_Order_Grid) { //Thanks Paul Ketelle for your feedback on this

            $block->addColumnAfter('sagepay_transaction_state', array(
                    'header' => Mage::helper('sagepaysuite')->__('Payment Info'),
                    'index' => 'sagepay_transaction_state',
                    'align' => 'center',
                    'filter' => false,
                    'renderer' => new Aardvark_Modifications_Block_SagePaysuite_Adminhtml_Sales_Order_Grid_Renderer_State(),
                    'sortable' => false,
                )
                , 'real_order_id');
        }

        return $observer;
    }    
}