Thanks for the offer of help - sorry I didn't respond straight away. I don't own the right to freely distribute that model so I tried to create an object that behaved the same way - without luck! I have arrived at a solution of sorts so I thought I should cap this thread. When I tried tackling other aspects of the project that needed work such as introducing more plants I found that the transparent area was not solid at all but was allowing the grey cloudy sky to show through the eye and head. This was in fact the opposite problem to what I thought was going on. I then decided to look at the material of the eye. The eye is composed of 2 spheres- 1) with the pupil and 2) a transparent sphere for the lens and glossiness. The problem was mostly solved by introducing a much higher level of refraction (1.52) , showing caustics and allowing a 10% fade to white on the 2nd sphere. There is still a slight artifact around the edge of the eyelashes geometry but its not too bad - compared to what it was like originally.

Cheers.