We recently observed a resonant spin excitation in Ba1-xKxFe2As2 (x=0.4), one of the newly discovered iron arsenide superconductors with Tc=38K, on MERLIN. This demonstrates that the sign of the energy gap is opposite on the disconnected hole and electron Fermi surfaces at the Brillouin zone centre and zone boundary respectively, indicating that the pairing potential is repulsive at short range. A subsequent MERLIN experiment showed that this excitation broadens in Q with increasing doping and becomes incommensurate at x= 0.7 when Tc falls to 20K. In this proposal, we plan to study the Q- and temperature-dependence of this excitation in Ba0.3K0.7Fe2As2 to provide a more detailed comparison with theory in a system with imperfect nesting.