.... replacing the starter motor and the crankshaft position sensor fixed this issue.
Just for info...the starter motor was replaced with a later model Bosch version for a M57N2 motor (12417798035). These are 1.8KW have 9 teeth and a 74 mm flange as opposed to the M57N starters (12417788680) which are 2kw and have 11 teeth and a 76mm flange.
Seems they are interchangeable and the later model is lighter and more efficient?

Thanks for the follow-up. :thumbup:

Did you end up replacing the glow plug controller and plugs too?

With all that, I bet it starts just by touching the key. ;)

The later version starter motor is lighter yes but less powerful like the stated power tells. It has less teeth because with lower torque it needs lower gearing. This is all because of manufacturers counting on cost and weight (consumption). First M57 had a 2,2 kW starter, M57N 2,0 kW and M57N2 1,8 kW.

If you still have the old starter motor open the back cover and most likely it's heavily covered in dust from worn brushes partially shorting it. In many cases these can be revived by simply cleaning them.