[Neil Jones]

One thing I always think about is how it was called The Demon Headmaster but he was actually only headmaster for the first 3 episodes. I think the books were much the same though.

The whole of the first book saw him as a headmaster (and the first three episodes of the first TV series, since that condensed the first two books into the show's first series.

The rest of the book series? Yeah, that was just his name. It was never revealed what the character's real name was, aside from it being a drawn out plot point in the final series of the original show, and even then it wasn't shown.

Character was obviously able to stand out on his own without a need to be a headmaster as such. It was more about the hypnotics than anything else.

2019 Headmaster is on the iPlayer and Hardiman's cameo is here (from 23:23):
www.bbc.co.uk 

"[2019 Demon Headmaster]: Every bit as creepy and terrifying as first time round even from an adult's point of view. No spoilers but if you saw the original show there comes a point where although it was tooted as a reboot it actually isn't. And that was fabulous. " - my post on TV Forum December 2019.