This post on a local history blog mentions M.R. James in passing, though that's not why I stumbled upon it. It gives a little insight into how often authors of supernatural fiction would have seen people dressed up and performing weird rituals, ceremonies etc. We think of the Order of the Golden Dawn as a bit outré and marginal, but it was arguably just a posh and pretentious version of what thousands of arty middle-class Brits got up to on a regular basis. Check out those Druids.