You may need to prove to the court that the address you have given the bailiff is the last address that you believed the respondent to have lived at. You may need to ask the court for clarification on the process regarding this. I am 100% certain (confirmed by a VERY expensive solicitor) that you can continue with the case without ever receving a response to the petition, and in that case it would go through on an undefended basis.
It may also be worth checking out www.192.com/ or even the online BT phone book. These can be really useful for tracing people and they are not going to bankrupt you with ridiculous fees.