The mural shows the passage from Malory's Le Morte D'Arthur, book IV chapter I:
"So by hir subtyle worchyng she made Merlyon to go undir that stone to latte hir wete of the mervayles there, but she wrought so for hym that he came never oute for all the craufte he coude do, and so she departed and lefte Merlyon."

Letter from William Morris to Jenny Morris June 13 1877 (the Collected letters of William Morris Vol 1, pub. 1984 p 377):
"...I think it must be 20 years ago since I went there with Aunt Emma. Haddon is the most beautiful of the places about there; and since it is not lived in by the grandees that own it and has nevertheless the roof on is in as good a state as one can expect an ancient."

This would bring the date of Morris' s visit to c.1857, the time of the Oxford Union Murals. At that date the Devonshire Hunting tapestries, now in the Victoria and Albert museum , were hanging in Haddon Hall, Derbyshire. There is therefore a slim chance that Morris encouraged Burne-Jones also to visit. The similarity of Merlin's headdress to those found in the tapestries may be evidence of a visit at a similar time. (See Linda Woolley "Medieval Life and Leisure in the Devonshire Hunting Tapestries" pub 2002. illus p 59)