Thanks for the reply Brendan,
Yep checked the collimation of the laser, all OK.
The primary was only moved up to achieve focus on the camera, as it is I have about 5mm of inward travel left when focus is achieved.
Looking by eye down the focuser the reflection of the mirror clips are near the outer edge of the secondary and centred as you see in most "how to collimate" tutorials.
I can't say I've noticed this effect in other peoples images, it is a little wierd.
JD