Thanks for the replies guys.
You're right Lindon. I do have that guide and I watched several Youtube videos too.
https://www.youtube.com/watch?v=8ROvNH5uwDo
https://www.youtube.com/watch?v=YAVGcGEBmCE
https://www.youtube.com/watch?v=8G98RTP6jbY
Plus a few others. They're what gave me the confidence to have a go. A little knowledge.......
Those Astronomy and Nature blokes aren't too bothered about dropping things down the tube!
Renato, I'd read about that offset secondary on faster scopes somewhere, but the Orion manual shows it as central in the assembly instructions. Those instructions are maybe a bit of a one size fits all approach as they do apply to several different focal ratio scopes. More reading to be done. That aperture mask process sounds worth looking into (hoho). I presume it has just the one hole?
Glen and Malcolm. When I removed the mirror carrier I marked its position but, I must confess, I did not mark the mirror position. Doh!
I will remove it again and look for the earlier mirror markings. I don't think I overtightened the clamps but I'll loosen them off anyway. I either got lucky putting the whole assembly bck together or there is no specific orientation of the various parts. I'll check it all again.
It's all good practice!
Thanks again.