No I hadn't edited the CSS created by CSSMW so maybe its a bug.
However, Yes, that sorts out that problem but causes another.
Now the width of auto appears to resize each individual drop down item to its automatic width, rather than have the resize to the largest item. It looks a mess.
For now I've set a fixed width which is far from ideal but it does appear that there is a fundamental issue with this and IE. I appreciate the IE is the gutter as far as compliant browsers go but we have to make sure sites work with it as the most popular browser!