[Arran]

I've always used regular paint and varnish remover (the kind that burns when you get it on your hand) to get stuff like poly off. Acetone will work but I find it evaporates too quickly unless you focus on stripping a small area at a time, not to mention stinking up the house and the fire hazzard. Regardless of what you use polyurethane is nasty crap to get off, it has a habit of just softening but not lifting off so it involves lots of work with scrapers, putty knives, and steel wool, and you need to keep it wet the whole time or it hardens again. In my opinion it belongs on wood floors and boats not on furniture, but I question it's usefulness on those too.
Regards
Arran