Thanks everyone that replied ! Yes, it was balancing. I took the car back yesterday and now it's (almost) perfect. What was interesting is that they only changed the weights very slightly.
+5grams on one, and the other just repositioned a little. Also, as a couple of people pointed out, the lack of a central hole in the Pug alloys does make it a bit more fiddly to do the balancing (and perhaps inherently less accurate if an adaptor is used?) The place did it right away, no waiting, and no charge.
Thanks again,
- Steve