Hi Sam, I know what you must be going through. I am expecting a baby from my Tunisian partner in July.
Hope goes well for you. If you go to the following website:
http://www.britishembassy.gov.uk/tunisia
On your left you will find UK Visa's- Information for Sponsors - Sponsership Undertaking Form.
This is a sponsor letter that you will have to add to the application.
Good luck, let me know how you get on.
Nicolette x