2.
Some pointers in such matters:
A. Mere non-recovery of service tax from client/s can not be valid reason for non-payment of service if same was otherwise due & payable.
B. Mere difference between turnover as per Form 26AS and ST-3 return - by itself - is not sufficient reason to invoke extended period to issue SCN. It is burden on Revenue to prove that there was suppression of facts with intention to evade taxes, fraud etc. on part of tax-payer so as to invoke extended period to issue SCN.
C. Generally specking, Revenue ought to carry out due scrutiny / investigation before issuing such SCN.
D. You will find many case-laws which are getting regularly reported on such type of SCNs and many of them are against Revenue.
E. Above views should be seen from limitations of non-knowing the contents of SCN, reason for non-payment of service tax, grounds of defenses taken both on merits of the case & limitation.
F. In such matter, taking a professional expert's help & assistance in preparation of reply to SCN, appeal etc. can make lot of difference.
These are ex facie views of mine and the same should not be construed as professional advice / suggestion.