Does the 'fallen' sloping stone and the gap supposedly left by it not effect the light diffusion and 'patterns' observed? Would they be different if the gap was filled and the fallen stone not there?[/quote]
It certainly does affect some of the observations, partly because stone 7 would have acted as a 'projection surface' for the light at sunrise coming through different apertures and moving across and down at different rates. This may have played a part in determining the furthest point of the Sunrise at Summer Solstice. It would also have created very accurate gaps, possibly for determining the Sun's furthest setting point at Winter Solstice (although it would need putting back up to demonstrate this!)
We believe Trethevy Quoit may be unique in having accurate alignments at both Solstices and possibly the equinoxes as well.