Dave

hi neil every thing seems ok to me but i would use balbond sbr on ply its better apd..thats more for softwood etc...prime the edges and back of the ply.. ( 12mm should be ok as long as the floor is fixed well before hand.)
dont forget to prime the walls as well.. bal spf takes aruond 16hrs to set..and will last in the bucket for about 5hrs..so no need to rush....