how to calucalte period

   1. service tax paid  20,00,000   for 2 months delyed- paid 20,00,000/-  after  six monts
we found  paid 20,00,000/- ok  but delay period of  2months not paid -

now what is the procedure after six months for delay period of  2 monhs interest or  till
 date of six months period  on paid amount.

2. party is not paying  Service tax Regularly. when we pay the  Service tax  then we are reimbursing . But we are paying intrest to Serice tax intrest to Department.is it right  or when we collect  from the party we pay the department with out intrest. pls advice