For years I have tried to remember the name of a children's programme that was on (possibly on CITV, but could have been CBBC) when I was a child in the UK in the 90s. It may have been a rerun of an earlier broadcast.
My memories are vague but I remember there being a small circle of large stones, akin to something like Stonehenge, that opened up and allowed the characters to enter. I believe there were adults and children who entered a sci-fi themed base through the stones.
I seem to recall my dad saying that the main character was played by someone who had been a pop star or singer when he was younger.