Cheers hydrophyte. Yeah, its Calathea rufibarba I think, or something similar (was just labelled 'Foliage Plant' when I bought it, very helpful!).
It took a few weeks to get used to being in water, and all the old roots rotted away quite quickly. New roots were well developed by a month or so I think, and there's now a big 12" tangle of them growing out of the planters.
The leaves were somewhat larger when I bought it, and have gradually died off to be replaced by these slightly smaller and less purple ones. It's now a very happy and robust plant though and growing densely.