This depends very much on where you live and who the social worker is employed by. If she is from your cmht, you can contact them or PALS and request that there be a change.
If from the local council, then you would need to contact their social services department and ask to speak to the appropriate person. For details of contacting your local council for this, see:
http://www.direct.gov.uk/en/Hl1/Help...ices/index.htm
I hope that is of some help.
Roiben x