Can someone please advise how I associate multiple testimonials (created via ModelAdmin) with a page. Below is the code for the page class that the testimonials should be added to (mysite/code/ServicePage.php). This shows a testimonial ListboxField, but it's blank and provides no records to add, even though I've created two via the ModelAdmin:

class ServicePage extends Page {

	private static $db = array (
	    'Intro' => 'Text',
	    'USPStripIntro' => 'Varchar(255)'
	);

	private static $has_one =  array (
		'Banner' => 'Image'
	);

	private static $has_many = array (
        'Sections' => 'ServiceSection',
        'USPs' => 'ServiceUsp'
    );

    private static $many_many =  array (
		'Testimonials' => 'Testimonial'
	);

	public function getCMSFields() {
		$fields = parent::getCMSFields();
		$fields->addFieldToTab('Root.Main', TextareaField::create('Intro','The introduction for the page'),'Content');
		$fields->addFieldToTab('Root.Main', TextField::create('USPStripIntro','USP Intro Text'),'Content');
		$fields->addFieldToTab('Root.Attachments', $banner = UploadField::create(
			'Banner',
			'The banner image for the page'
		));

		$fields->addFieldToTab('Root.Sections', GridField::create(
            'Sections',
            'Sections for this page',
            $this->Sections(),
            GridFieldConfig_RecordEditor::create()
        ));

        $fields->addFieldToTab('Root.USPs', GridField::create(
            'USPs',
            'USPs for this page',
            $this->USPs(),
            GridFieldConfig_RecordEditor::create()
        ));

        $fields->addFieldToTab('Root.Main', ListboxField::create(
            'Testimonials',
            'Selected testimonials',
            $this->Testimonials()->map('ID','Name')
        ),'Content');

		$banner
	        ->setFolderName('page-banners')
	        ->getValidator()->setAllowedExtensions(array('jpg', 'jpeg', 'png'));

		return $fields;
	}

}

class ServicePage_Controller extends Page_Controller {

}

And here's my code for the Testimonials, which are created via a ModelAdmin (mysite/code/TestimonialAdmin.php):

class Testimonial extends DataObject {

    private static $db = array (
        'Name' => 'Varchar',
        'Role' => 'Varchar',
        'Company' => 'Varchar',
        'Testimonial' => 'Text'
    );

    private static $belongs_many_many = array (
        'Pages' => 'ServicePage'
    );

    public function getCMSfields() {
        $fields = FieldList::create(TabSet::create('Root'));
        $fields->addFieldsToTab('Root.Main', array(
            TextField::create('Name','Client Name'),
            TextField::create('Role'),
            TextField::create('Company'),
            TextareaField::create('Testimonial')
        ));

        return $fields;
    }
}

Managed to solve this by changing the testimonials call in the class to:

$fields->addFieldToTab('Root.Testimonials', new CheckboxSetField(
	'Testimonials',
	'Selected testimonials',
	Testimonial::get()->map('ID', 'Title'))
);

Only minor preference would be to make this a ListboxField as opposed to CheckboxSetField, but how do I modify the above to achieve this? Obviously tried changing the CheckboxSetField reference to ListboxField, but this seemed to output as a dropdown, as if it were a $has_many, not $many_many.

`ListBoxField` has a common ancestor with `CheckboxsetField` (`DropDownField`), so you should be able to drop in `ListBoxField` where you have `CheckboxSet`

Tried changing it to the below, but still not working. I get the 'Sorry, there was a problem handling your request.' message:

$fields->addFieldToTab('Root.Testimonials', new ListBoxField(
	'Testimonials',
	'Selected testimonials',
	Testimonial::get()->map('ID', 'Title')->toArray()
));

setMultiple should do the trick:

        $fields->addFieldToTab(
            'Root.Main',
            ListboxField::create(
                'Testimonials',
                'Testimonials',
               Testimonial::get()->map('ID', 'Name')->toArray()
            )
            ->setMultiple(true) ,
            'Content'
        );

[EDIT]: saw you already noticed the toArray() bit :)

You are using 'Name' though, not 'Title'

This works perfect, thanks martimiz!