Thanks Guys,

Seems I have hit a very rare snag from the comments on here and the OA site.
Just my luck - at least it only cost me a new battery before I found it.

Kendo - no oil leak from rear of engine would have got through the oil seals on starter motor housing to cause this amount of gunk - it was oil tight when I split it and then it started to leak once the long bolts were relaxed. So it ain't got in there by external drips or splashing.

Got to have come from inside engine OR something greasy/ oily breaking down within the SM - which I would guess is unlikely.

Currently researching buying a used one to refurb or biting the bullet for a new Mitsuba unit.