Thanks for all the responses! :)
I'll keep it to supervised play for now. Just to delve a little further, what do you all suggest when they are interacting (under supervision) and the play gets too rough. Opinions seem to be conflicting here and I am open to all ideas.
For a little more detail, the older dog always instigates the play and it is always just play and quite gentle. But it will often escalate to a point where the adult dog jumps on the pup, grabs it around the neck and pulls it around etc. I stress the fact that there have been no injuries to date and it still seems to me like overly rough play. I'm sure if it were more than that the older dog would have done some serious damage by now.
What are the suggestions for intervening here at the point when it goes too far?
Thanks again