Might you be from my party this Saturday .. I'm guessing so!
The eyehole is about 50cm above the normal sump opening. It is a narrow slit that you access by swimming across while the sump is opening. Alternatively you wait a little while longer and the levels drop to the larger space beneath the eyehole giving you, in low water conditions, a pleasant stroll along a little stream and, in higher conditions, a somewhat wetter walk!