Add text to Order Confirmation emails (Text email only)

Being a UK store I have to provide customers with a copy of their rights under the Distance Selling Regulations in a durable medium. This means that just having it on the website is not enough.
The simple solution is to include it on their order confirmation email.
Inserting the relevant text in email_template_checkout.html file above the footer seems to be the easiest way for HTML emails.
I am at a bit of a loss as to how to add it to text emails. I would very much appreciate it if someone could point me to the correct files to edit to insert text only in the text version of the Order Confirmation email.

Many thanks.

/includes/classes/order.php
See the send-order-email function

Or just edit one of the existing phrases already in the confirmation email, and add your extra text to that phrase.

The fast way to find out which specific file to edit, especially for existing text, is by using the Developers Toolkit in your Admin:
http://www.zen-cart.com/content.php?80-how-do-i-find-out-what-files-to-edit
http://www.zen-cart.com/content.php?81-how-do-i-use-the-developers-toolkit

Thanks.
I' shall give it a go and report back.

I've defined the statement in includes\languages\english\checkout_process.php as EMAIL_TEXT_DSR
Where should I insert in /includes/classes/order.php?
I tried adding below
//addresses area: Billing
$email_order .= "\n" . EMAIL_TEXT_BILLING_ADDRESS . "\n" .
and it was inserted into the email but it made the email double up - everything was duplicated .

I added below
$email_order .= PAYMENT_METHOD_GV . "\n\n";
and it broke the checkout.

Ideally I'd like to add it just above the disclaimer.

Thanks.

If you want it just above the disclaimer, then:```php
$email_order .= EMAIL_TEXT_DSR . "\n\n";
// include disclaimer

That's the one!
Many thanks.