Hi Moth

Working late, I see. The plan is to erect a trilithon, similar to the five that are near the centre of Stonehenge. These are self-contained and consist of two uprights and one lintel, unlike the outer sarsen circle where the lintels share an upright with the next one.

The lintel will be raised on a wooden crib right next to the two uprights. When it reaches the appropriate height it will be levered sideways onto the uprights, which can be "adjusted" if necessary by ramming extra rubble into one side or other of the base hole.