Long Meg.

The first thing is...

All that fuss about Stonehenge and I'm parked just at the edge of the ring, on a single track, off a narrow road

We began our expedition after coffee and cake at The Watermill cafe and farm shop:

The Watermill,

Little Salkeld, 

Penrith, 

Cumbria. 

CA10 1NN.

Finding Long Meg isn't too difficult, there are sign posts.
If you turn right when leaving the Watermill, follow the road around to the right, go over the cross roads. The little road is a left turn. It leads to a farm, so you will need to turn around when you leave.

Below is a diagram of The Sanctuary, close to Avebury.
The Sanctuary was connected to Avebury by a double row of stones, the Kennet Avenue. These ended at the North-West, outer ring of stones.

William Stukeley made drawings, before all the stones were destroyed, or ploughed up.

Long Meg stands in the South-West.
If Long Meg marks an avenue entrance, it is in the South-West (as seen from centre of ring.) not the North-West, as at the Sanctuary.

The Sanctuary marks the South-West, with a recumbent stone.
In the case of Stonehenge, this position may also  have been marked with a recumbent stone, the 'alter stone'.

The Sanctuary.

The South West is most obviously the direction of midwinter sunset.
It is the direction to see midwinter, evening Venus.

In the summer morning, the solstice full moon sets here.

It is impossible to know as yet, if Long Meg is an outlier stone belonging to a different, previous version of the monument (like the carved stone outside Bryn Celli Dhu) or if it was one of many stones marking an avenue.

And the question remains, how the importance of geographical, and astronomical alignments fit together.

One possible fly in the ointment in further research is this:

"Beneath the stones are the remains of the gypsum / anhydrite industries of the Long Meg Plaster and Mineral Co. Ltd.." Reference...