What did your instructor say about it? Was it a bumpy day or not?
In still air, IF the aircraft is trimmed properly (both pitch and yaw - if you don't have yaw/rudder trim you might need just a tiny bit of pressure on one of the pedals) the aircraft should be able to fly hands-off for minutes at a time without course or altitude deviations.
Perhaps you were too busy navigating, looking outside and such that you were not quite trimmed correctly, and/or you were unknowingly putting a little pressure on the yoke/stick. If that's the case, try taking a little more time to trim the aircraft for hands-off flight and make sure you're not putting pressure on the yoke/stick when it's not required.
I'm not advocating that the aircraft should be flown hands-off all the time, though. But it's a good test to see whether you are in trim or not. And being in trim is the first step to smooth flying.
For the PPL skills test standards, check out this document, particularly Appendix 3:
http://www.caa.co.uk/docs/33/Standar...19_A%20v6_.pdf