COPD shows that the grid ref in the monitoring form is for Cumberland. The ref given for Wapping is 2939 5749
The entrance is in a small opencast feature in an alcove at the back RHS that appears to go nowhere. Closer inspection will reveal the back of the alcove is a masonary wall with a green warning notice about the site being a SSSI and no rocks are to be taken. There is a small gap at the top of the wall, with a bigger drop on the inside. Climb over this with care and have fun