In the end I just removed it as follows:
The starter motor is the cylindrical object on the right hand side of the bike, just behind the cylinder head.
1. Using an alan key (hex hey) disconnect the starter motor cable from the solenoid. (The solenoid is positioned just behind the battery.)
2. Pull the cable out - it runs past the battery, under the tank and then down to the starter motor.
3. Release the 2 short bolts holding the starter motor to the engine (and not the two long bolts that run down each side of the starter motor which hold the starter motor together).
4. Firmly pull the starter motor sideways until it pops out. The motor is held in quite tightly by an oil seal and so it may take some persuasion to move. I used a thin long piece of timber pushed up against the starter motor casing from the left hand side of the bike and tapped it gently with a hammer to push the starter motor out.
5. Once loose, carefully remove the motor - the motor inserts into a loose cog which will probably come out with the motor. Make sure the cog doesn't fall off and go bouncing around in the dirt, and take notice which way round the cog goes - it has a flange on one side - the starter motor side if I remember correctly.
Once removed the starter motor can be taken apart quite easily. Just take note of the orientation of the end covers. Clean out the carbon dust, emery cloth the commutators, remove old grease and put in new grease on the cogs and you're good to go. When putting back together, I found that it was easiest if you put the axle into the housing first, and then put the plates which mount the cogs into the housing. This is because when I tried to put the axle in last, it would jump when the magnets took hold and the washers on then end of the axle jumped out of place. There are (apparently - from a youtube video I saw) 3 washers at either end of the axle, and should be in the order of a thin one, a thick one and a thin one. I say apparently because I only saw 2 washers when they fell off.
Putting the starter motor back in is the reverse of taking it out - gently tightening the two short bolts - each a turn at a time to keep the motor straight, make the job of getting the motor reseated easy.