Solent tools provide replacement carbon brushes (see link below)
http://www.carbonbrush.co.uk/index_files/boschlawnmowercarbonbrushesrotak.htm
I have also had a problem with my Rotak 40 but need to replace the plastic brush holders - unfortunately Solent do not stock these. I have tried to improvise a repair but unfortunately my 'new' holders only maintain position for a few seconds.
I have looked at buying a replacement motor but it is prohibitively expensive - £170.99 from Currys Partsmaster. A brand new Rotak 40 can be bought from Screwfix for £89.99. It's a bit dispiriting when you make attempts to repair something but it turns out cheaper to dispose and replace it!
Ricky K
October 2010