The reason I was not seeing the attributes within the category is that they are the content and I did not specify current content when doing the export. It is a slightly confusing area.
For a normal object, like a document (let's say a word doc), you might consider it obvious to export the content. You might also want to export the attributes that have been assigned to it. So you would do ... xmlexport&content&version=current&attributes.
For a category, the attributes that are defined from the "content" of the category object in the same way that a document contains the word file as content. So a Category contains Attributes - but could also have a category and attributes APPLIED to it - and this is what the &attributeinfo parameter would attempt to export.
So what you need to export is .... .../livelink.exe?func=ll&objId=91263&objAction=xmlexport&scope=sub&nodeinfo&versioninfo=current&content=base64
Hope that this helps anyone else.
OS