Look what I've got!
It's not finished yet. My welder mate had extreme difficulty with the welding, and as a result of the porous original magnesium and new repair of aluminium being different metals with different cooling rates, he wants me to add additional strength holding both together. He suggested I fill the inside of the repair with fibreglass, and overlap beyond the line of weld. We think about an inch and half would be plenty. He also suggested I buy Liquid Aluminium to fill welding imperfections. I'm having a look for that online now. If I can locate something suitable locally, I'll get into that job tomorrow afternoon.
My fellow had a hard time working this piece, and he's a professional welder who owned a large engineering works. If it gave him grey hairs, imagine what it would have done to me! I'm ever so grateful for what he has achieved. The next step is up to me now.