Just for the benefit of other people with this problem....
After Ray's last comment I recreated the menu bar from scratch with all the original code but the problem still persisted so I decided to create a basic spry menu and then use the 'if' command to tell my pages to display this menu if viewers were using IE6.
Guess what, this had the same problem! I looked on the Adobe forums and there are load of people writing in with issues with menu bars displaying in IE6.
One of the 'experts' commented that very few people use IE6 anyway so I looked at my web stats and found that only 2% of my visitors used IE6 so I tend to agree.
I have therefore given up on this issue as the menu works in every other browser.