October CMS Custom Mail Layout

I'm struggling to get October CMS to work with custom mail layouts via code. Under Configuration section it states we can use subject and layout.

https://octobercms.com/docs/services/mail#mail-views

I can see the subject is working as expected, however I'm struggling to get any form of layout coming out - its simply chucking out the HTML within the template.

Here is the register.htm template file which is located within my plugins views/mail directory:

subject = "Registration"
layout = "default"

==

Dear {{name}},

Thank you for registering you interest...

Yours sincerely,

Support

==

<p>Dear {{name}},</p>
<br>
<p>Thank you for registering you interest...</p>
<br>
<p>Yours sincerely,</p>
<br>
<p>Support</p>

And I'm calling it like so:

Mail::send('pluginname::mail.register', $data, function($message) use ($data) {
    $message->to($data['email'], $data['name']);
});

Make sure you have registered the mail template in the Plugin.php file so it can be bound to the layout.

public function registerMailTemplates()
{
    return [
        'pluginname::mail.register'  => 'Register template.'
    ];
}