Thanks Allan, that's really useful.
I've noticed you pointed out that Tableau uses WGS84 in a reply to a previous post of mine a while back, but at the time I don't think I had got my head around it all!
I personally don't think that it's communicated very well by Tableau how to get geographic data in other projections into the software, although I may have overlooked something. Perhaps it's because it's a geography/mapping issue rather than a Tableau issue.
Either way, hopefully this post will shed some light on the problem if anyone is having the same problems I had.
You're very welcome.
One of the good things about the Tableau community (I've certainly found) is that if you are trying to do something, and you get stuck, someone usually has figured out the problem before. I've learned a lot from folks like Richard Leeke and Joe Mako; I guess part of my involvement here is "paying it back".