Maybe the solenoid is not pushing the bendix out to mesh in with the flywheel ring gear, so try tapping it with a stout object, ie piece of wood/small hammer when someone it turning the key.....Plan your escape route 1st. If that does`nt work, the starter motor needs removing and you may have a build up of crud on the shaft of the starter, which stops the little geared cog, (bendix) from sliding freely.
Dip in parrafin/diesel, tooth brush, clean up and put a little bit of light oil on shaft, Mike.