Thanks for the info Cytan, I really value your time and input on this
I've seen that page you linked before but had forgotten about it. I've always been wary of playing with the primary mirror collimation screws (in fact I've never done so) there is various conflicting advice on the forums about them and indeed the original collimation 'manual' from Astro-Tech for the GSO RC says never to adjust them at all!
Anyway I got a break in the clouds and got a few minutes to do some more testing.
* This time I used Polaris and initially centred it in the OTA/sensor.
* Some wind, seeing not brilliant.
* Collimation at this point is just with the Cheshire that I did to correct it after my first attempt with the Tri-Bahtinov mask previously reported.
Here are 3 successive images at the centre, nothing adjusted between images:
* Collimation at the centre looks reasonable
** though you can see the differences in successive image without any adjustments that I mentioned previously.
** The 10-4 orientation looks slightly off
* The traditional Bahtinov again doesn't appear to show any miscollimation, and rotating it (on a centred star) doesn't show it either.
* At the corners, there does appear to be issues, and it alters in different corners, though the 10-4 orientation is still the main culprit.
I was battling clouds so had to call it a night, so this has just been an information gathering evening, no tweaking tonight
I didn't get a chance to take an airy disk image. Though thanks for the tip about APT, I've downloaded the free version and will try the collimation checker tool once I get an image.
I'm still pondering about the interpretation, any input greatly appreciated