After several attempts at a fix, finally found the good advice here:
http://www.defusion.org.uk/archives/2007/04/02/when-overflow-hidden-doesnt-hide-in-ie/
The advice was:
So in my case swapping that out for position: absolute; on the child and overflow: hidden; position: relative; on the parent worked a charm. Now IE 6 plays nicely like its other browser friends.
In my case, position:relative could be left on the child div so all containers and child have the same relative positioning. I used this with the jquery animate function to slide a panel of content text into display area of new client website (GY).