The spanner I made worked a treat for holding the nut and getting the old bolt undone. However I could not for the life of me get the nut started on the new bolt, so was after much frustration and swearing ended up giving up and sailing one bolt shy. Grrr

I am going to weld the nut onto a rectangular washer and grind a point on the new bolt to make it easier to locate the hole in the nut.