Please refer to para 3.0 and 3.1 of CBEC Circular o 207/5/2017-ST dated 28.09.2017.
Extract of the relevant portion:
3.0 Payment of service tax on or after 01.07.2017 as a consequence of detection of evasion or any other circumstances.
3.1 The issue will arise only in the case of assesses who were not registered under ACE. It may be recalled that in the registration module of ACES, there is a category of "non assesse registration." This may be used to obtain registration and make payment of service tax. Any difficulty in this regard may be resolved with the office of ADG, Directorate of Systems and Data Management, Chennai.
However, would like to check whether the turnover was less than 10lacs pa in those years. If the value of services rendered is less than 10 lacs pa then the question of registration of does not arise. Register only if the value of services rendered in any year is more than 10 lacs.
Under the GST, the threshold is 20 lacs pa.
Hope this clarifies.
Regards
S.Ramaswamy