EDIT: This may not be relevant for Paul:
The nut can just slip with perfectly standard and correct components. I've experienced this, not every time, but ever since I had my first chainset with removable rings.
There is a special tool, a back spanner, to hold the nuts still, but I never use one.
Keep turning the bolt and the nut will eventually stop slipping as contaminants disperse. Press the back of the nut hard with your finger (and never mind the pain!).
Whenever refitting the nuts and bolts, degrease everything including the chainring and your hands - except for the bolts threads if you wish to use a small amount of grease.