Please refer to the following provisions before discussion:
As per Sec. 68 of the Finance Act, 2012,
Every person providing taxable service to any person shall pay service tax at the rate specified in section [66B] in such manner and within such period as may be prescribed.
As per Sec. 66B of the Finance Act, 2012,
There shall be levied a tax (hereinafter referred to as the service tax) at the rate of twelve per cent on the value of all services, other than those services specified in the negative list, provided or agreed to be provided in the taxable territory by one person to another and collected in such manner as may be prescribed.
As per Rule 2(e) of Point of Taxation Rules, 2011 (POT Rules)
"point of taxation" means the point in time when a service shall be deemed to have been provided;
Considering the above, here is the reasoning:
1) As per Tule 4 of the POT rules, if the service is provided prior to the change in effective rate of tax, then the point of taxation shall be date of payment or issuing of invoice, whichever is earlier. Hence, the service is deemed to be provided on 4th April 2012, considering rule 2(e) as above.
2) Since as on 4th April 2012, the rate of Service Tax is 12%, the Service is chargeable at 12%.
Please revert if this logic does not suffice.