Julian Cope presents Head Heritage

"Of course you can see the sun set behind Amos Hill , if you stand right next to it . But it is impossible to see it from the line of stones at Thornhope , which is where you suggested , no matter how much you brainstorm it will never be true .
You don't need GE to work that out ".

your GoogleEarth has deficits, on a major scale.
Because the stone row is lower than the top of Amos Hill - even after its slumped a metre or so - you are looking up at it and the horizon is actually the field behind the hill, at the centre, with Black Hill rising to the left. During spring and autumn the sun, from the stone row, sets behind Black Hill. At the solstice - and a few days before and after - the stting sun moves out from the obscuring Black Hill and appears to roll down its edge. Which has a steep edge at about NY662532. I've never seen it properly, just half, in half out, on the nineteenth.

There is a fallen stone beyond the row, which I assume was an outlier, and that seems to be the best spot to watch these summer sunsets from as the view from the row itself is impeded by a stone wall and scrub trees.