procedure for bifucation of service tax

Suppose a company providing services has been registered by its branches for service tax i.e. for each branch has its separate service tax no.. Now my Q is if company takes services of advertisement company which raises one bill in the name of Head Office where no services are provided only billing are done. Further how will company divide such bill to take service tax credit among its branches.
Plz reply and specify in detail what documents company will have to maintain?