Penalty for late payment - Only Interest or even penalty?

Hi,

Do we even have to assess and pay the penalty of Rs.100 a day, if the service tax is paid up with in a month after due date.

Just paying interest of 15%pa would be sufficient or even penalty is to be paid?

If so, do we need to inform the same to the Superintendent about it in writing or not required.Because the ST3 form is even asking for 'Source Documents' if these interest and penalty columns are filled.

Kindly clarify know, whether penalty to be paid or not as I was confused writings, at one place penalty is not required for late payment and only in case when ST due is still not paid, penalty is applicable is written and at other place both are applicable is mentioned but only 25% if paid within one month.

Can this 25% can be directly calculated by the assesse and be paid or only after writing from the department?

Thanks in advance.