Interest rate for service tax

According to notification No. 14/2011-Service Tax dt. 01.03.2011 and 15/2011-Service Tax dated 01.03.2011 in which also the rate of interest is increased to 18% p.a. from 13% p.a. The new rate of interest of 18% will be applicable from 1st April, 2011.

But in case if the company  has defaulted in making service tax payment relating to FY 2010-11, & is willing to pay now along with interest.

Then the interest rate applicable shall be 13% or 18%?

Thanks & Regards