How about this:
sage: E=EllipticCurve('75a1')
sage: P=E.heegner_point(-56)
sage: P1 = P.point_exact()
sage: K = P1[0].parent()
sage: E = P1.curve()
sage: G = K.automorphisms()
sage: def apply(sigma, pt):
....: E = pt.curve()
....: return E([sigma(c) for c in pt])
sage: sum([apply(sigma,P1) for sigma in G], 0)
(0 : 1 : 0)
sage: r56 = K(-56).sqrt()
sage: sum([apply(sigma,P1) for sigma in G if sigma(r56)==r56], 0)
(47/14 : 363763/587699796992*a^7 + 363763/41978556928*a^6 + 2404005/2998468352*a^5 + 80502545/10494639232*a^4 + 49062308611/146924949248*a^3 + 138252123705/73462474624*a^2 + 31053597881/18365618656*a - 9514261313/4591404664 : 1)
That last point is defined over Q(sqRT-56)):
sage: tP = sum([apply(sigma,P1) for sigma in G if sigma(r56)==r56], 0)
sage: tP[1].minpoly()
x^2 + x + 20007/2744