im not sure your chippie sounds very competent . he should have used the underlay upstairs .the reason the shop sold it to you is that it helps with sound transmission and makes the floor feel softer underfoot. also the floor should ideally be glued not nailed as nailing is not spillproof, loosens over time and makes it very awkward to take up afterwards
battens plus ply plus floor = 2 inches off all your doors and possibly cutting the seal off the bottom off of your hall door and a ramp down into your kitchen.
for a fuller explanation why i think battens are a very bad idea read here to see why
http://www.askaboutmoney.com/showthread.php?t=67919