Yes, it sounds like an episode of UFO, but it isn’t.
My parents took me to see it at the cinema, and neither my mum or dad were into science fiction, or space, so I suspect it was a special treat for me. I remember how thrilled and excited I was – it really made an impression on me (apart from the title, obviously!)
Having half-heartedly searched for it on numerous occasions, having a vague idea that it was called something like “Moonbase” or “Moon 3-2-1”, I did a more scientific search today, and discovered the film is Countdown. Moreover, it is an early Robert Altman film, staring James Caan and Robert Duvall. Amazingly, it is available from the States on DVD, and even though the reviews of it are lukewarm, I have to get it, to see if it was really as exciting as I remember it.
It is also based on a book – The Pilgrim Project by Hank Searls, so I might see if I can track that down, too.