You do not have to offer a pension to your PA(s) at the moment.
However, the UK Government introduced a new law in 2012 to help people save more for their retirement. This means that all employers will have to offer a pension to their workers if they are not already in one. Very large employers are doing it first; other employers will follow from 2013.
As PA employers will fall into the small employers group it is unlikely that most PA employers will have to offer a pension until 2017.
At the moment, auto-enrolment will only apply to people aged 22 and over and to people earning over £9,440 per year, which may exclude a lot of PAs as many work part-time hours and so do not earn over that amount.
For more information, contact The Pensions Regulator.