Richard,
I'm by no means an expert or particularly experienced but, what you show in that image does not fit with what I understand coma to look like.
My understanding of coma is that stars in edge of fov will have little comet tails where as your image shows almost a double star or if I was imaging, what I would call a star trail from imperfect PA & tracking.
I'm wondering if you have some other distortion going on here rather than excessive coma?
It might pay to check your primary mirror & see if your mirror clips are a little tight.. pinched optics can induce aberrations too....
Again, no expert but, you comment on using a farpoint laser for collimation then using the cheshire to 'fine tune'. You should really be using the cheshire for collimation with the laser to 'fine tune'.
An excellent guide to collimation can be found here:
http://www.astro-baby.com/astrobaby/...ian-reflector/
It's a common misconception that a laser is the be all and end all of collimation & in truth.. whilst it will line up your optics it can still result in poor colliimation... have a read of the guide I've linked to, it describes a process of getting optics aligned.. firstly with your focuser since this is the reference point required for viewing...
Apologies if any of this is 'suck eggs' material, I don't mean it to be... it may just be that your aberration is not coma but, rather another issue that can be fixed without additional expense & it might be worth checking some things & perhaps revisiting your collimation process before you part with your hard earned cash...
