Used some of this info to start looking at replacing our foam mattress so I thought I'd post up some findings to keep youze up to date.
Went to Makin Mattresses (Canberra) and they were pretty informative and knew what we were talking about. They basically make you a low profile inner-spring mattress by adding $50 to the price of the mattress you like in the store. We looked at the basic range from $250 to $499 and the guy said the model with the leaf pattern ($399 I think) is popular for this purpose (though we're going on the road for a year and liked the firm $499 one, of course
).
They basically give you a mattress with half the spring height so that the feel of the mattress is otherwise the same, including double sided so you can turn it over. Measures 170mm (just under 7 inches). Takes a few weeks to make it up.
Heard Sealy make low profile mattresses so we to Snooze but they had no real idea what we were talking about and had to make a few phone calls....
Waiting on a quote from 'made to measure mattresses' to compare - will post their quote here once I get it and let you know who we go with.
It's not as hard as I thought it would be to get a low profile mattress - thanks to all for the past info!