1. Input credit taken will need to be reversed, and even though not utilised, interest would be payable from the date of availing the credit.
2.There is no time bound limit for availing the credit, however as an advise it should be availed within one year
3. you have not clarified when the credit was availed, whether on 04/08/11 or 01/10/2011. Normally credit would be availed only when bill has been received. If you have aviled 0n 04/08/11 then interest will be payable barring 3 months from 25/11/2011 i.e interest would be payable from 04/08/11 till 22/09/11
4.Interest is payable @ 18% for wrong availment from the date of credit till the date of payment. If intervened by department equivalent penalty may be levied