I've just read the Wittenham Clumps TMA entry, found and followed Rhiannon's link to the stunning painting.
In the write up on the information card in the Walker Art Gallery's version ('Landscape of the Moon's Last Phase') of this, it mentions that Nash did many paintings of the site towards the end of his life (using field glasses to bring it closer!).
I wondered if anyone knew of any more paintings by this artist of this site?