I have also noticed that the new starter motor sounds much nicer. Now I can compare the old one did sound a little rough.
g8rpi - The issue only happened when the car had stood for approx 3-4 hours after being warm. I would start car cold mostly fine in the morning to do school run and get to work for 9am. Car would cool until lunch time at 1pm. Car really hard to start takes a lot of cranking and sometimes I would think it was not going to. It always would eventually with big cloud of white smoke (indicating that its not a fuel issue). Pop home and park but engine is warm by now. Get in car to go back to work for 2 pm, car still warm starts no trouble at all. Park at work 2 pm. 5pm leave work, car again hard to start as its been allowed to cool for >3 hours, white smoke. I thought that the hotter the SM the less current it would draw hence after a few cranks the draw would reduce enough to allow the ECU voltage to be high enough to operate correctly. Same goes for when car is really warm not cooled for long, plus in both cases the engine would have warmed or be warm enough to reduce the draw due to less friction. I just cannot explain the mainly easy starting when stone cold as I would expect this to require even more amps due to the extra friction, cold everything. I can only think that some sort of spike occurs at these temps that help it work. Glow plugs I suspect would have a different impact at different temps too.
Inspecting the old SM the is no obvious signs of damage. e.g. high wear, movement in the bearings.
I am sure that the reason is down to the ECU unable to operate due to voltage dropping too low when cranking on a faulty SM which weirdly is really bad at a certain temperature. Another hint to this was that on occasion when starting was bad I would get an excessive engine heat warning flash on the dash after it had been stood for >3 hours which is obviously incorrect. As you say not an obvious cause but I do know that all ECUs are very fussy about voltage and that it will crap out if voltage is too low. Same goes for PC CPU overclocking. It is sad though to see so many post like this on car forums with dealers who cannot solve it and have changed so many other expensive parts. Many describing the same symptoms as i had almost exactly.
Anyway again I hope this information helps someone avoid a lot of pain.
Cheers