I'm thinking ahead to next year when we start fitting out a new bathroom. I'd like to build a wash basin stand/cupboard covered with travertine mosaic tiles.

Any advice on the best material to use? Marine ply?

Thanks for that link.  This one from their website is quite close to what I have in mind...

http://www.wedi.co.uk/vanity1.htm

It looks a bit like the blue polyurathane insulation I'v had from Sheffield Insulation before - I've used it to build model airplane wings.

I was going to use a vessel type basin with the taps mounted on the vanity unit so I'd have to figure out how to mount the taps without crushing the foam.

Take a look at the price of Wedi, it's pretty expensive stuff, I would use 3" x 2" wood and clad it in Aquapanel.

If you really want to use an extruded foam (like Wedi) then go for Dukka board it's a lot cheaper!