Here's a plan of the site showing the alignment's general direction:
Aerial view of Winter Solstice Sunrise alignment
Here's a view along the alignment, taken in November 2011 from the west side of the monument.
Stone 58 is the left upright of the trilithon immediately behind the nearer set of three stones (which are part of the sarsen circle's perimeter). The notch is visible about 5' above ground level.
Long view along the alignment from the west
When trying to determine the position of Winter Solstice Sunrise in 2500 BC, we have to allow for the change in the angle of the Earth's axial tilt since then.
These days it's 23.44°, back then it was 23.98°. So the sun's maximum and minimum declination today is ±23.44°, and it was ±23.98° 4500 years ago.
The effect of that difference is to shift today's Winter Solstice Sunrise position along the horizon to the left compared to where it was when the sarsen phase of Stonehenge was constructed.
We can accurately calculate the Sun's declination (in our era) for any date, so all we need to do is get some photos close to Winter Solstice when the sky is clear.
Here's the view through the notch in Stone 58 taken on the morning of December 10th 2011 shortly after sunrise.
There's an electricity pylon on the horizon at the extreme right hand side of the 'window' formed by the notch in Stone 58 and the northern edge of Stone 53 in the background - it's a useful landmark.
View along the alignment December 10th 2011
Knowing the declination of the sun when the photos were taken, the apparent size of the solar disc (about 0.5°), its declination at Winter Solstice in our age (-23.44°) and in 2500 BC (-23.98°) allows us to create an image like this (several photos have been overlaid to give the track of the Sun for measurement):
Determining the position of Winter Solstice Sunrise in 2500 BC (click for a larger version)
Update: 22nd December 2011
Amazingly, we had an almost clear horizon so here's a composite picture of winter solstice sunrise for comparison with the image above. I'm pleased my calculated position was pretty much spot on :-)
Winter Solstice Sunrise 2011 - composite image taken on the Stone 58 notch alignment from the back of Stone 8
Here's a picture to give a sense of what it actually looks like when the Sun is shining straight down this alignment (I had to crouch down slightly to get this shot as the Sun only aligns after its risen in our era, not exactly at dawn).
Sun shining along the alignment, January 2011
But this final image says it best. I'm fractionally off the alignment to the south as the photo was taken on the 6th December, but even so this was a spectacular sight.
Dawn, December 6th 2011
Update 22nd December 2012
The beautiful clear sunrise of this year's Winter Solstice meant I was able to capture images of the Sun from first gleam through full orb while standing on the alignment near the west end of fallen Stone 8.
Here's an animation of sunrise, followed by an updated version of the explanatory diagram from last year. Now that I have two series of sunrises to overlay on the same horizon, it's possible to refine the position of sunrise in 2500BC.
The inset image in the top left of the following diagram recreates the view of full orb at winter solstice sunrise in 2500BC as seen through the notch.
Determining the position of Winter Solstice Sunrise in 2500 BC - revised 2012 (click for a larger version)
I'm deeply indebted to Gordon Freeman, whose book "Canada's Stonehenge" published in 2008 first set me on the trail of trying to confirm his ideas concerning the notch in Stone 58.