Alasdair

i dont know how long the trunk is as there are no pictures, but i would sujest you attempt an air layer at the graft point (ish) well, more of a layers. Stripping away the bark and using a root homone liquid with built in fungicide to stop it becoming infected etc. Ive found that layering can produce some superb nebari, many roots in all directions. Then, after a few years for the roots to develop, depending on the branches, you should have a champion specimin of a maple!