When were those stones moved?

From the above (click to enlarge) we can see that there are several candidate glacial episodes during which the erratics (and the Stonehenge bluestones?) might have been transported by ice of one sort or another. Look at the right-hand curve on the top diagram -- you will see that the cold (glacial) stages are shaded, and coincide with Oxygen Isotope stages 2,4,6,8,10, and 12. The most likely episodes are the Wolstonian (c 200,000 years ago) and the Anglian (c 450,000 years ago). The latter was a BIG glacial episode, with much more glacier ice in the Northern hemisphere ice sheets than at any time since. Rohling and colleagues think that there is very good evidence that global sea-level at this time was around -140m. That means that in order to maintain a relative sea-level somewhat equivalent to that of today (ie with the shoreline more or less where it is today on the edge of the Channel Coast landmass), there must have been c 140m of glacio-isostatic depression in Southern England and along the Channel Coasts.

See Rohling et al

Magnitudes of sea-level lowstands of the past 500,000 years
E. J. Rohling, M. Fenton, F. J. Jorissen, P. Bertrand,
G. Ganssen & J. P. Caulet
NATURE, VOL 394, 9 JULY 1998