MASSIVE MASSIVE thanks to @max_damage and @jockywilson11 for your advice. It might not look like much but the image is a shiny new starter motor and the sound is the engine running after starting on first turn of the key. Replacement fitted in under an hour and that included some head scratching and air filter removal due to a different (improved) bolt setup on the replacement starter motor. YES YES YES BACK TO MODIFYING IT!! This was with original wiring - I wanted to test it on that before installing the new power cable too.