Floor Tile Decoupling Membrane

We're about to lay about 60 sq M of floor tiles over a suspended floor in out holiday cottages. Now the floor is 9"x2" joists, and although it was waterproof flooring chip board we've ripped it up and are replacing it with 18mm ply to give as rigid a floor as possible. For peace of mind I'd prefer to put a layer of decoupling membrane, but it's unbelievably expensive - about £450 for that area. (unless someone knows a more economic source). What is the panels experience of tiles over a suspended floor? The room I'm in at the moment was conventional joists and chipboard, tiled over without a membrane, and the tiles were ok for the five years before we refurbished, and again we laid 18mm ply but as it was a small area I put in a membrane.
AWEM
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On 26/01/2013 11:30, Andrew Mawson wrote:

odds are tiles will crack.
I have a Silent Floor engineered I beam floor system, with glued & screwed deck Used a special 2 part flexible adhesive .... tiles still cracked
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On 26/01/2013 18:01, Rick Hughes wrote:

The bathroom "I" did 6 years ago has been fine. Stiffened joists up with noggins every foor or so and then screwed 18mm ply down. Then put electric heating mat down covered with epoxy tiling compound then 300x300mm ceramic tiles.
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On Jan 26, 11:30 am, "Andrew Mawson"

You can buy special flexible tile cement for wooden floors. No chance of putting some additional support underneath to be belt and braces?
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