Turns out the connection from the earth terminal,to brushes was broken. Autospark has done a "repair" . Did the job and got it moved back into the garage at least... part is ordered.

The brush holders are steel and are rivetted to the ally back plate. On early ones the whole current goes though the rivetted joint because the earth bolt is on the ally back plate. Corrosion gets in between the steel brush holders and all back plate and causes a bad conection and overheating - mine melted a bit. As a quick fix I took it apart, cleaned it and put 3/16 pop rivets in, that was a year ago and its still working great. I bought a new back plate to fit and noticed that the new ones have the earth bolt going though the brush holder plate so the current flows straight from the brush holders to the earth cable. This is also a good reason to use the starter earth bolt not one of the fixing bolts (i use both).