.... 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?