You can certainly paste a URL into the "Message" in the Email Receipt, it is not formatted as a clickable link, whether it was clickable or not would probably depend on the email client, but the user can certainly copy and paste the URL.
Thanks Josh, yep already done that. I was hoping to create a clickable link, but as far as I can see there doesn't seem to be a way from a non-html email using the email receipt. You're right in that the email client might pick it up and I've tried entering as an <a href= but doesn't seem to pick it up. I'll try a few other email clients to see if it works in those. Thanks, Kevin