Yes ,magnetic north is a bit off from grid and true north but the monument provides a big target from any one of the car park post sites ( approx 7 degrees of the horizon ). So any loss of accuracy with the compass is surpassed by that problem .
That is encouraging, as it means that it is possible the pine post holes could be used to locate midsummer, autumn, midwinter, and spring in the annual cycle of seasons. However I will try to obtain a precise position of the sunrise in late December – weather permitting, and take photos of the positions of the sunrise and the Stones.