Well done Cambrian that's correct, did Ken Dibble's book help? Yorkie was hot on your heels there but you were able to get it correct.
It was in the hollow opposite Bryn Ifan ( John's Hill) that is the last property on the left as you go up Bryn Y Bia Road. Across the road nowadays is a small lay by with a road going uphill and that is where Pant Yr Ellyll was.
The clues were there:-
Hollow = Pant
Fiend = Ellyll
The photo is on a leaflet at the Conwy Archives