If anyone is having the same problem as me with this bolt, here are a few notes which may be of help.
On the Rover 75 Diesel Auto, there is one nut (15mm) & bolt (13mm) located on the Radiator side of the starter motor. Once you have removed the engine accoustic cover, the air ducting, disconnected the battery and the cables from the starter, this nut & bolt is easily removed. As RoverRattles stated, the second bolt goes into a threaded hole in the starter motor, the bolt head (13mm) is located on the bell housing side.
Looking from above the engine, between the inlet manifold and the battery box, you will clearly see a 15mm bell housing bolt head, it looks a little bit too far around to be fitted to the starter motor and it is! The bolt you need is approx 2-2.5 inches from this one, on my car it was located underneath a metal water pipe (if you were looking from the passenger side of the engine bay, the easily visible 15mm bell housing bolt head is approx just before 12 o'clock, the starter bolt is at approx just before 11 o'clock).
To remove the bolt I needed a 13mm socket on a 9" extension bar, I also unhooked the water pipe (the one with the vent valve) from their clips around the battery box, this allowed me to reach the extension bar with a torque wrench and get enough movement to undo the bolt. I removed this bolt from the starter, but did not withdraw it from the bell housing so it made refitting easier. Note, this bolt has approx 25mm of thread beyond the bell housing, so it does take a good few turns to fully disengage from the starter.
I refitted the starter using the nut & bolt finger tight, I could then just turn the extension bar I had left in situ to locate and tighten the 13mm bolt.
Sorry this is very wordy, I took a couple of photos and followed the instructions in the FAQ, but it appears I do not have the options to upload them.