Hi Guys, thanks for your replies, I have heard back from Nikon and here is the first paragraph of their email:
Thank you for contacting Nikon Support regarding the D300 light leak.
I am sorry to learn of the problems that you have been experiencing with your Nikon equipment. It seems that your camera has developed a fault that has to be addressed at our Service Department. Please note that the turnaround time is generally between 2 and 4 weeks - depending on part availability, testing requirements, etc. Also, we are not able to guarantee the repair will be carried out on warranty conditions until the camera is physically inspected.
My camera is well outside the warranty so I wont follow it through. I did some test today, using the only two ND filters I have which are a ND2 and an ND4, I stacked them together and set my 70-300 lens to f32, the ISO to 100 and pointed it at the sky, nothing shows up, the exposure was moderately short, around 10 secs (from memory). I took some more with the cap on and black masking tape around the AF lamp, I used it sparingly at first, to see if the light was coming in from one portion of the AF lamp housing, a very thin strip which I repeatedly moved around for each exposure. I always ended up with the blue artefact. I found that even the slightest opening through the tape caused it, but if I masked the lamp completely it was fine. So I have concluded it is definitely a problem with how the lamp is seated. I am wondering if there is a seal around it that is possibly pinched or badly seated.
Anyway, thanks for replying, Rich.