Or a carefully positioned narrow cold chisel and tap it anti clockwise. What is the diameter of the round section of that nut?
23mm... I found Laser 3464 Emergency Wheel Nut Remover 4pc covering.
Sizes: 17 | 18.5 | 21.5 and 26mm x 38mm long
Should do the trick... handy for rusted or rounded off nuts in the future...
Cheers for the help...