I've been trying with the help of wikipedia to identify the first Doctor Who episode I ever saw, but TBH, I'm baffled. I'm reasonably certain it was a Pertwee story, but could've been Tom Baker. It's definitely one of those two. I don't recall the plot or the main aliens, all I can definitely remember is that there was some sort of military force running around the usual quarry in Wales wearing black uniforms with red trim.
Does this ring a bell with anyone? As I said, pretty sure it's Pertwee, but reading the episode guides I've been able to find hasn't turned it up.