I've created a View that displays the activity of users with which the current user has established a 'follower' user relationship (using the UR module). So, visitors would be looking at user X's page and seeing a feed of what user Y has done, because user X is a follower of user Y. The feed works - so far so good.

But I can't control the 'Items to display'. This parameter seems to be applied to ALL the activity of all the users. So whilst I'm trying to display the 5 latest activity items by followed individuals, I'm displaying the latest 5 activity items and then filtering out all those that are not owned by followed individuals.

Am I missing a filter setting here?

The View takes an argument of 'User ID' as the 'requester' user. This is then connected to the Requestee user with a 'Requestee' relationship (which is in turn connected to the Heartbeat activity message owner).