I suppose the main question is: given the type of power line in the location of the sighting, which I'd humbly suggest is the feed running uphill to the Crest Mawr reservoir, is it possible for there to be some fault which would cause arcing for at least 6-9 minutes (the duration of the video). Obviously as there was a storm going on at the time, we have a plausible explanation for why a fault might have developed in the first place.
We can't really blame the witnesses for misinterpreting what they saw, as it was something very much out of the ordinary. But it seems the two boys decided very quickly that it was a UFO, and this no doubt also influenced the two other witnesses (and the dog, who probably barked as his owners were agitated). The problem is that investigators are often quick to reassure witnesses that they're not seeing things by confirming their belief that they saw a UFO, rather than reassuring them that it's OK to misperceive stuff.
By the way
@Sid didn't you have a really spectacular sighting yourself once?