Cheers Ray
I had been steering away from longer subs as I don't have an lp filter in the train. The astrodon filters though seem to do a fairly good job of cutting out some if the worst of it so will push the subs longer and see where it gets me.
More likely to be limited by blooming with this CCD before I get anywhere near 15 minutes on broadband imaging
Will let you know how this goes
Thanks again