Our head developer managed to help me get it working. We're still not sure why but this seemed to work
protected function _prepareCollection()
{
$quoteId = $this->getRequest()->getParam('id');
$quote = Mage::getModel('sales/quote')->getCollection()->addFieldToFilter('entity_id', $quoteId);
if ($quote->getFirstItem()->getId()) {
$collection = $quote->getFirstItem()->getItemsCollection(false);
}
$this->setCollection($collection);
return parent::_prepareCollection();
}