If anyone has an unserviceable Bosch starter motor or parts please see the Wanted section of the forum as I need a replacement brush holder plate.
Did you try this?
If mine was playing up, I would give one of these a try…
Brand new, £58 delivered to your door.
Edit, if you need any hints on stripping and rebuilding, there is a great explanation on This Old Tractor …
I see the motor on eBay is now listing at £73.50 but since Brexit it is also likely that HMRC will bang on an import tax.
My local auto electrical supplier is ordering me an upgraded replacement for the Bosch at £72.