Starter removed and stripped. All looked OK, no obvious problems and clean. However, during removal, I noted the securing bolts were not as tight as I would have thought they would be. On removing the starter, I noticed the face of the flange of the motor and the mounting face on the bellhousing were covered with a fine film of oil. Both faces were cleaned with meths, the starter replaced and bolts tightened up satisfactorily. On starting a vast improvement in cranking speed over previous. I can only assume the starter motor earths to the engine through its mounting on the bellhousing and this was compromised by slack mounting and oil on the flange faces