I have tried the mig welder and nut approach, the stub sheared another 7mm in, at the point where the tread enters the block. I then used the guide I had made to center drill the stub and left it to soak in the WD40 penetrating solution overnight, soaking from the front and back of the threads I have now given a No4 Dormer stud extractor a lot of abuse, the stub is not moving at all. There is some damage on the mating surface of the top fixing and the engine mount bracket suggests that the top bolt was doing all the work. I think at some point in the past the lower bolt was tightened up and seized before it was fully seated. At the moment I think a Helicoil is the only option I have left.