Updating for information and help others in the same situation.
Method: set the default text in the single line text field to:
{http_referer}
Make the single-line text field invisible.
Do not change the code as machforms gives it, in other words, leave it as:
data-formurl="//www.website.co.uk/forms/embed.php?id=85343
The resulting notification email will give the URL of the form it was sent from.