Porcelain floor tiles on plywood

[AliGage]

Just to add, I might be wrong in my thinking, but if the few alternative questions you've asked above are cost related. The difference between the two is approximately £3.50-4psm. Assuming you would also glue the Ply Overlay in your option.

Small price to pay in comparison to a failed floor if incorrectly prepared.

 

[AliGage]

@Raz out of interest have you had any survey the premises and spec the floor from a tiling perspective.
400sqm is quite an area. Your timber construction would still require expansion consideration imo. Your tiled surface would require expansion joints not just ditra. Why is this structure 700mm off the floor? What's going to be under it? How will the structure be supported? Rather than sqm what is the length and width of the floor Space? Presuming it's a square of course.

It's one thing to ask how to correctly prep a timber substrate but it becomes a bit more complex when the scale is as big as yours.

 

[Rizzle from the Portizzle]

ok you have suspended floor pree 1930 would be my guess the joist shoud have surport pillars over long spans open up a few spaces so you can get under the floor and check these are still holding now consider the wieght loading going on top now check the distance between them if the are more than 3.5 lm bridge the middle with another surport now thats done using 8x4 ply work out the midlle of there joints and nogging out with 4x2 now lay and screw down 12,5 mm 5 ply now soak that in wood glue then cross ply with 12.5mm ply screw down when the glue drys it should be as hard as nails .get you self a glass of waterhalf full place in around the floor and walk past all ways checking for defletion the more the water move the higher the deflection now if you have no movement you have got it right now heavy traffic will require a none conpresive over lay iso matt will do the trick or hardie backer its your choice

 

[AliGage]

Superb post @Ray TT @ Porcel-Thin