I live in the UK and my daughters father left us when she was 7 weeks old for another woman. He left his eldest daughters mother and now the other woman is pregnant! He is registered on her birth certificate as her father and she does have his surname but she has hardly any contact with him and he pays no maintenance. I know I would have to have his permission to formally change her surname to mine but can I change it unofficially without going through the courts?

You can find information here:
Name Change by Deed Poll - Can I change my children's names? (http://www.ukdps.co.uk/CanIChangeMyChildrensNames.html)
And here
http://www.adviceguide.org.uk/index/your_family/family/change_of_name.htm

/Changing a child's name without the consent of an absent father whose whereabouts is not known
It may be possible for a mother to change her child's name by Deed Poll without the consent of a father (who has parental responsibility) who is absent and his whereabouts is not known. Usually, this situation arises where a mother wishes to change the surname of her child because the child has the father's surname and the father is now absent following separation or divorce. The mother may have entered into a new relationship and is using her new partner or husband's surname or has reverted to using her maiden name.

If the father has parental responsibility, is absent and his whereabouts is not known, you can apply for a Deed Poll but you need to support your application with a letter of consent, which should include information about what reasonable measures you have taken to contact the absent father - for example, writing to the father's last known address and contacting relatives and friends of the father etc./