Ok, I think I have fixed this!
Last night I had a look at the actual measurements between the spider vanes and they where definitely not square! So after I used the adjustment bolts, I managed to get them inline with each other and square. It was cloudy, but I managed to find a very distant bright light and focus on that, and the diffraction spikes where almost perfect! I also think that the field is evenly illuminated now, although I had some trouble testing that.
Now all i have to adjust is the bolts in the tube and it should be performing nominally.
Thanks heaps guys
Dan