Long Exposure, Purple Cast Update

In the last blog I raised the issue of a purple colour on a long exposure shot taken at the Giant's Causeway (see here).  So today I decided to see if I could find out the cause of this problem (thanks to Keith who also helped). The following sequence shows the variability of the problem given different settings:

As you can see a slight touch of purple in the first image, as I widened to 16mm the more noticeable it becomes, but it gets very obvious once the aperture is closed down to f/22. So what was the problem, at first I thought it was light leakage through the "distance window" so I "gaffer taped" this.

But that didn't solve the problem.  The problem is light leakage through the Eyepiece, once closed with the Shutter it was gone.