I saw this series on Australian TV in the early-mid 80s, but it could have been made in the 70s. It was definitely British. It had a small number of episodes, possibly six or eight.
As with a lot of this type of show, the plot involved a kid or kids on holidays trying to solve the mystery of WTF was going on.
What I remember:
- The mystery involved Celtic lore and a hammer that would give the wielder immense power.
- There may have been an older guy who was an expert, possibly a Professor, in Celtic lore.
- One of the locals was also an amateur expert in the local lore, possibly he was a druid in a cult. His day job was gardener or gamekeeper or something similar. He was the main antagonist.
- Towards the end of the series, in order to gain the power of the hammer, the antagonist and another man (possibly the prof?) engaged in a duel. Before the duel was over the antagonist picked up the hammer and declared himself the winner. When his opponent objected that that was against the law, the antagonist declared "He who holds the hammer makes the law!" Turns out he was right and the good guys had a collective "D'OH!" moment.
That's not much to go on, but over to you...