What was that book - early 80s horror set in North Wales?

[hugegadjit]

Hi all, wonder anyone can help - remember reading this about 1983, bloke gets sent to Snowdonia for work, he's (I think) something to do with hydro electric power, there's some occult goings on up there in the mountains, a local idiot savant called Dewi and a local witch (bit of sexy business goes on with her, that much stuck in my mind). What might the book have been?

Thanks in advance