OK so I have solved the problem of PayPal returning to page 1 of the form after payment. Simply by setting up a custom page. There is however still a problem in that if the user decides to click the Cancel and return to merchant button without making payment, it still returns to page 1 of the form.
Why can we not add a custom failure page as well as a custom success page? Certainly this is possible if you set up a PayPal form manually. Perhaps someone at App Nitro could suggest a workaround for this by editing a file but it would also be great to have this facility as an option in future updates of Machform.