The photo/map shows the exact location. It is still open land, though now under the ownership of Thames Water. If you stand at the first lay by on the on the A30 westbound past the Crooked Billet r/bout the field on your left is the site.
The memorial could quite easily be situated at this lay by.
In Barbados I saw a similar memorial for a Cuban DC8 that crashed offshore, and likewise in an earlier thread there is one for the Air India 747 off the Irish coast.