If I send a form out to a user, is it possible that they can complete part of the form.
When they have completed their part is it possible for them to send it to someone else to complete the rest.
I know it's possible to save a part completed form and have the link e-mailed to carry on, is this the only way?
Yes, this is the only way. FormsCentral doesn't provide a work flow for form filling but users can utilize the "Save Web Form" feature to accomplish this.