Thanks Adam
The example worked but the new version didn't fix the problem. However, it did push me in the right direction and so for the sake of community spirit I thought I'd let you know that I found the following worked....
I had a "position:relative" style applied to al <ul> in the wysiwyg stylesheet. Removing this one line fixed the problem, although I have no idea why this should have changed things.
Cheers
Dan