Many thanks, both.
This does seem to be possible with layers, but it's certainly complicated! I think I could probably get it working, but trying to edit it in the future is going to be a nightmare! It's a shame that Xara WD's navigation bars won't work properly on pop-up layers. I really liked WD when I first started using it, but as I'm building sites with it, I'm starting to see a lot of rough edges. :-(
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