Replacement Bathroom Floor

[Red]

I've decided to replace my bathroom floor prior to underfloor heating and tiling.
What is the best and most cost effective way to do this?
18mm ply or chipboard followed with tile backer board?

would 18mm ply on its own be suitable I know you shouldnt tile straight onto chipboard.

Any help appreciated, Thanks in advance

[aeromech3]

I am about to do some cloakroom floor tiling and found that when you read the instructions on adhesives it gives a good idea, for wood, flexible and water proof is what you need, not cement based; perhaps an expert can advise on sub floor

[aeromech3]

Well update, the adhesive I got is also cement based but highly modified : Keraquick Rapid, it does say can use on ply etc for floors but also suggests using Mapei Flexible Plus which costs as much again, for better flexibility / adhesion; now for a heavy tile on a wall I can understand the adhesive bit but if the substrate is solid why need more flex (Abroad, my builder just used a fine mortar on a concrete screed and no probs so far after 3 years).
The Keraquick is dark grey and yes it does stick to everything including my newly primed skirting (need a lot of handy dry rags) it also dries out quickly so a slightly sloshy mix is easier.
D*m those videos make it look so easy, in my case I ended up with great clods of it on my spreader and trowel and when I tried to flick it back in the bucket it ended up on wall paper and my feet haha!