OK cool thanks chaps...
I see what you mean about the soft focus Daniel, I had focused it with live view rather than the bahtinov mask
I've tried some longer exposures recently (4 minutes) and it only seems to be the brighter stars that get a bit of a blue halo. Interestingly, it seems to get diluted during stacking, along with all the other colours!
Will look into a filter too, I think the reducer/flattener has a 2" filter thread scope-side.