I know this reply is a bit late for you but it may help someone reading this,
tiling straight onto the boards in not generally a good idea (you may wish to take them up in the future and it may mean the whole floor coming up)
buy yourself some 2mm hardboard, cut it to the shape of the room (I did my en-suite cut from one piece of the stuff) and using 1/2 inch screws ( so you don't go all the way through the floor boarding) screw the hardboard down making six inch squares of screws ( i.e each screw 6 inches apart in a row, each row 6 inches apart) this stops the floor from moving anywhere. Use flexible adhesive.
I have just pulled up my tiles from the en-suite and it nearly came up in one piece as i could pull the hardboard off the screws, then I had to go round and remove all the screws but nothing is easy in DIY.