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Andrew, have you tried putting an eyepiece in an examining the defocused image either side of focus? Sometimes you can get a feel for possible sources of these issues. Might also get clues applying slight pressure to main or secondary mounts and see if star distortion changes on a bright'ish star.
I'm sure its not your issue, but I used to get very similar pattern of star distortion on my non-photographically optimised 8" Newt - the bottom of the focuser protruded down into the light path of the main barrel.
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