It is also rather difficult to write intelligent criticism of this show because it has all already been said. My own long-standing criticism is that I find it confusing - even to the extent of not being able to disentangle episodes, adventures, series, who is who, and so on - and this is not helped by a changing cast of Tomorrow People.
There is also the matter of this being a children's show, and I have been trying to think myself into how a child would view this episode, and I suspect the emotions would be a mixture of envy for the Tomorrow People and horror at how the enslaved boys are treated. Isn't that the point of much writing for children that we are supposed to be on the side of the goodies? The drawback is that the goodies here are the next step in evolution so we can't have their powers.
Trying to think differently about this has made me reflect on what I was first thinking about it: I was thinking that the point that the diamonds are mined in dangerous, slave conditions and the way the Tomorrow People don't want diamonds mined in those conditions leads to a very grown up and uncomfortable conclusion. Actually that's how jewels and precious metals are always mined - mines are dangerous places at the best of times and the people who own mines tend to want to maximize profits. This uncomfortable fact is glossed over for the kids, but it does show that this is a show which can be watched on several different levels, always the sign of quality television.
This episode therefore encapsulates the show's best and worst features. My attempt to watch this through my own eyes as a child has distracted me from how unfortunate the episode is through adult eyes in terms of slavery and costumes which look like fetish gear. You would really have to be very innocent not to get the kinky undertones and the show is made more ambivalent by the fact that Mike Holoway was actually a heart throb of the time. Granted this is with the benefit of hindsight but it is even more unfortunate that he does a Jimmy Savile impression. I actually wrote to Savile myself but fortunately he didn't fix it for me.