SilverStripe Blocks - Block with contact form example

ContactBlock.php

<?php

class ContactBlock extends Block {
	
	/**
	 * Template accessor for the form
	 **/
	public function ContactForm(){
		return $this->getController()->ContactForm();
	}
}

class ContactBlock_Controller extends Block_Controller {
	
	private static $allowed_actions = array(
		'ContactForm'
	);

	public function ContactForm(){
        $fields = FieldList::create(array(
            TextField::create('Name'),
            EmailField::create('Email'),
            TextField::create('Organisation'),
            TextAreaField::create('Message')
        ));

        $actions = FieldList::create(
            FormAction::create('submit', 'Send Enquiry')
        );

        return Form::create($this, 'ContactForm', $fields, $actions);
    }

    public function submit($data, $form){
    	// process form data as usual 

    	// ...

    	// redirect
        return $this->redirect($this->pageLink() . '?contacted=1');
    }
}

ContactBlock.ss

<div class='$CSSClasses'>
	<h3>$Name</h3>
	$ContactForm
</div>

How do you get the HTTP GET parameter within the controller or the template? Like the ?contacted=1 part?