The grotto at Leeds Castle, Kent may hold claim to being the newest grotto in England. It was added as a tourist attraction to the site in 1987 along with a yew maze.
The maze sits over the grotto and the entrance to the grotto tunnel is in the centre of the maze - functioning as an exit route / reward for sompleting the maze. It is normally electrically lit with low level, atmospheric lighting. It has a number of designs reflecting classical designs used in grottos over time including a Grek monster Typhoeus and representations of earth, wind, fire and water, The Green Man and other figures.
The grotto is currently closed for repair but on my recent visit the gate in the centre of the maze was unlocked and open and the tunnel could be easily accessed but the lighting was all off.