Section 73 deals with recovery of Service Tax not paid/levied/short paid or erroneously refunded. The general period for issue of notice is 18 months and in the event of fraud, collusion, mis-statement or intend to evade ST the department can invole the extended period of limitation up to 5 years from the date of detection of such irregularity. In this connection, the general principle is that the proof of burden that the service provider / Assessee not paid the duty under the circumstances stated therein under such proviso, always on the department .
Though you have stated that the department noticed certain irregularities in your query, the details of irregularities are not mentioned and in the absence of that it would be difficult to give any adivce whether the assessee (LLP) is attracted the said provision.
Further, if you are confident that the irregularities noticed by the department are not covered in the situations of fraud, collusion, mis-statement or intend to evade ST etc., you can challenge the notice/letter if, any given justifying that such irregularities are not fall under the ambiguity of events attaracts invokation of extended period.