Service tax interest computation

Hi

One of the clients has calculated service tax output @ 10.3% instead of 12.36 %, my queries :

a.) Should the difference be computed by removing 10.3% say if invoiced is 30000, St is 3090, shoul i back calculate on 30000 only the differnce of 12.36 - 10.3 ?

b) The party has paid 10.3% only on september 12. So should theinterest for 10.3 % be calulated till that date & for remaining viz., 12.36%-10.3% till the due date of payment.

Kindly reply with respect to service tax provisions