The bit about the prone sarsen found at Bulford is interesting. It had been raised upright at one stage, but the horizontal pit from which it had probably been originally prized was close by (and mirrored it in shape).

So....
"This would indicate that sarsens were distributed over Salisbury Plain as well as the Marlborough Downs 20 miles to the north"

Are they implying the Stonehenge sarsens were local? Are all the original extraction pits awaiting discovery?