Water UFH Best timber floor base - Tile Advice

[themorg]

I'm about to start on an upstairs wetroom. There will be wet underfloor heating going in via spreader plates. The former will be a 22mm maxxus. Current joists are 7" at ~420 centres. 3x3m room.

As all the floor boards will be getting ripped out I'm unsure what the best material to put back down is. My current options are as follows:

1. 22mm No more ply (£370)
2. 18mm WBP ply with a 4 or 6mm cement board (£170)
3. 25mm WBP ply with a mat (£160 - ??? dependant on mat, suggestions also please)

Looking for suggestions on the best option (money is a factor, so the price of no more ply instantly raises the hackles, and it's not available local) or alternatives please.

 

[bsc ceramics]

option 2 is fine. make sure its well screwed down