when you say ''ex''.....have you been married or just living together with the father of your child??
sorry this is the first time I read about ''letter of abandonment'' from the biological father???... is it legal document ?? what it says in the document?? who will sign it?? will it be notarzed by the lawyer??
and with regards to ''permission'' , if the whereabouts of the father is unknown how can you let him sign the consent to join you to apply for settlement??