Time to do a landscape cross section to see if they are truly intervisible - I reckon they almost certainly are.
... well, it seems my hunch may be right.
Taking the centre of Stonehenge as 51d10m43s N, 1d49m35s W (from my GPS reading in the stone circle in 2000) then the bearing between that and a high point chosen arbitrarily on Sidbury Hill's southerly flank works out at 48d27m58s using the formula for the initial bearing (on a great circle route between the locations) at http://mathforum.org/library/drmath/view/55417.html .
That's close to the azimuth of sunrise at the time Stonehenge is supposed to have been built c 2450BC.
I'll need to do more calculations to be sure, but I'm fairly confident that if you were to stand at Stonehenge 4500 years ago and look along the Avenue towards the sunrise point at the summer solstice, you'd see the Sun rising out of the top of Sidbury Hill, 8 miles away.
Tracing the path on an OS map reveals a number of trackways exactly along this alignment, which have defined the edges of more recent tree plantations.
Please note! I'm talking about SiDbury Hill, which is a natural hill with a late bronze-age/early iron age hill fort on its summit, and is 8 miles north east of Stonehenge, and not SiLbury Hill which is an artifical mound 16 miles north of Stonehenge, near Avebury.