S.t.liability in case of renting of immovable property

One of my client being an Individual has given Factory premises owned by him (i.e.Commercial Property)to 2 occupiers being manufacturer covered under Excise law based on Sq.yard rate by means of Leave and Licence Agreement. Rental Income comes Rs.163000/- and 10000/- p.m. respectively from these 2 occupiers. Hence up to Aug,2012 the Total Rental income from each Individual occupier comes to Rs.815000/- and Rs.50000/- respectively, Hence total aggregating Rs.865000/-. Now for the month of Sept,2012 Rs.163000/ and Rs.10000/- If we consider the said Rental Income then My client has to register with S.T. Dept. and charge S.T. as total Turnover exceeds Rs.900000/1000000/- as the case may be. Accordingly I apply for the same i.e. ST-1 in the month of Oct,2012 i.e. on 25/10/2012 today itself.. Now my query is as under:- i) How to prepare Invoice/Bill to each Occupier along with Service tax @ 12.36% from Sept,2012 & On-wards as in Sept,2012 itself turnover exceeds Rs.1000000/- i.e. Rs.978000/- & Rs.60000/- from each occupier. ii) How to compute Interest @ 15% p.a. for delayed payment of Service tax and on which amount as well as for how many months i.e. either for 1 month or 2 Months.