Laibility of service tax if freight included in invoice

Sir,

One supplier has supplied chemical in their own tanker to our factory in the year 2011-12. The have not issue any consignment. The supplier charged Freight in excise gate pass after cst. Consignor has not indicate us that who is responsible for service tax.

We are registerd as GTA (service receiver). Please let us know who is responsible for service tax. Please reply.