Service tax on cold storage after negative list of services

hello sirs..

one of our client is engaged in cold storage activity. Earlier they were exempt from levy of service tax as they are only storing agriculture produce. But recently they have started storing non agriculture produce also. After applicability of negative list of service tax regime, they went for service tax registration and asked by department to pay the tax on their turnover from july, 2011. Please let me know how the tax liability will be calculated and also the calculation of interest on it...