Hey Darren,
Thanks for your inquiry! The only way to complete a form offline and submit later would be to have the client app installed on the device, or to use a service like Formotus.
Formotus allows forms published to your SharePoint site to be filled out on mobile devices even leveraging mobile features such as camera capture through specialize controls.
Please let me know if you'd like more info. or a demo.
Best,
Bre