Hi, Have set the ISO to 100 on my D5100 but have noticed on several photo's that auto ISO has then "over-ridden" the manual setting, mainly overriden to ISO 200 but sometimes 1200 and I think once 6400 (?) is this normal? This has happened both in the "A" and "S" modes
Yes, it's normal. The camera always tries to get the best exposure so, if you alter this vai Shutter speed or aperture then it will use ISO to get the best exposure. On some cameras you can lock the ISO to the value set but it might not give you the best exposure.