This from a Google BBC source:

The Truth

What actually happened was rather more prosaic.

Yes, the Metropolitan Police were called in - although their mobilisation was delayed until they had been sworn in by a local magistrate (otherwise they had no powers of arrest and could only act in self-defence, as with any other citizen). And there were Hussars - which evokes images of dark men with swords and horses, but this was an ordinary line infantry regiment.

The Met and the army were there because there had been riots and intimidation by the striking miners, and very considerable damage to property. The situation was judged locally to be out of control.

The night of the massacre was in 1910 or 1911 depending on your source, but probably it was one of the nights of rioting in early November 1910. November 8 is the likely candidate because...

On November 8 1910 during serious rioting, during which all but two shops in Tonypandy had their windows broken, the police staged what is now known as a baton charge. With truncheons drawn they advanced on the rioters. Some miners and police were wounded and one miner suffered a fatal blow to the head probably from a police truncheon

And that's the massacre. One rioting miner.