Even if the connection is within T5 it can take some time to move between the A, B, and C sections. And when your plane from Milan lands, the gate for the JNB flight may not be posted yet, so you would have to head to 5 A, gomthrough security, and then wait there for the gate to be announced.
We once almost missed out US-bound flight because the train taking us from 5B to 5C was delayed. We were waiting on the platform for 10 minutes, and heard the "gate closing" announcement for our flight. (They announced it way early, but we did not know that). Quite a panic, and when we finally got off the train we sprinted through the terminal to our gate, only to find they were still boarding.
Our last trip through Heathrow, in August, we were delayed a good 20 minutes at security because I foolishly did not remove my iPad from my backpack ( I did not see the advisory sign). I was referred to the secondary inspection area where there was only one agent working and quite a queue. My fault entirely, but those of us waiting felt we were being punished for our transgressions. We had a full 2 hour connect time and that was barely enough.