Very nicely done Matt.
I'd perhaps lower the black point on the histogram to increase contrast and really make the colours pop.
Focus slighly out but only just.
I suspect the flare is from your longer exposure subs, perhaps you can mask out the flare using the shorter subs.
I'm assuming you were imaging at F10?
Glad you used ISO 400, better result on M42 than using ISO 800 with the same exposures.
