I have been asked to prep a flagstone kitchen floor for laying lino. I was planning on getting away with laying 9mm ply to do this as the kitchen has the floor units in place. Was wondering what to use to fill the uneven areas, which I assume will be greater than 3mm over most of it. Would like to avoid screeding it totally, and thought that the ply would save me the trouble. Was planning on fixing it mechanically (screws&raw-plugs) but this is quite labour intensive. Any other suggestions?