It's pretty unusual to see a policeman dancing at a gig, but it's even more unusual to see one moshing.

And it's even more unusual than that to see one moshing at a Slipknot tribute act's gig - but that's precisely what happened at the Sixfields Rock Festival which took place in Northampton at the weekend.

A policeman who was on duty at the brilliantly-named Slipnowt's set also turned out to be a big fan of the masked American rock bands and not only knew the words to some songs, but joined in on percussion duties when one of the band left the stage.

Taking his truncheon from his belt, he used it to bang a beer keg held aloft by a man in a clown mask. Pretty standard.

Watch it below (click to play):