Sounds like that's the one to use then, assuming I can get someone to ship it to Dublin, but that's another thread altogether...
Just to be 100% sure I've got it, here's my new list:
1. 18mm WPB ply (not marine ply)
2. 12mm tile backer board (I can only get 12mm rather than 10mm)
2a. Electric underfloor heat mat
2b. Latex
6a. Isomat or similar, taped on seams and at walls
7. Tile adhesive
8. 8mm Slate Floor Tiles
9. Grout
10. Slate Tile sealing solution