In manual mode the iris can be closed to F9.5 The next stop is close.
In bright conditions 9.5 is not enough to retain say 1/50 shutter.
Now, in auto mode, when I press the exp.lock button I see iris levels all the way down to F22. How come I can't do this manually?

Per, I cannot give you a straight answer to your question. However, you'd want to avoid closing the iris much further than f/9.5 due to diffraction effects anyway. Instead, use ND-filters - if necessary an additional one to the one built into the stock lens. Here's an interesting article on iris-induced diffraction effects:

http://www.luminous-landscape.com/tutorials/understanding-series/u-diffraction.shtml

Incidentally, I've been thinking about buying a variable ND-filter. Although expensive, such a filter would enable me to shoot at the aperture/shutter speed of choice most of the time, independent of lighting conditions. Does anyone have experience with variable NDs?

HTH,

Ron