Agree with Dave bathroom floors are a job for a membrane like Stik2 Durabase CI or Stik2 Durabase WP, the beauty of using these products is they act as a bridging mat and a waterproof membrane.
Where you may think your floor doesnt need tanking the bridging element of the mat will stop your tiles cracking if the floor beneath should move so it is well worth using. If you check out
wetdecs or the
Tilux site you will find a full system the Tilux shower base former and the Stik2 matting work hand in hand and are made for the job.