From the photo they look like Robinson R44 helicopters (or similar, I'm no expert on helicopters, just done some web searching).
I think the R44 is a light and relatively cheap helicopter used, amongst other things, for training. After a bit more searching, I've discovered that Helicopter Services Limited of Wycombe Air Park offer "Mountain Flying in North Wales - two-day weekend event", so maybe that was what it was.
(Or else it was some very rich mine explorers!)
Dave