So I just restacked the Carina Nebula frames with the cold/hot pixel detection boxes unchecked.
The donut/bubbles have gone. Wahoo!!!
The focus of the subs might still be a fraction out but i'm well pleased with how this turned out, considering it was only my second attempt at a proper astrophoto through my telescope.
Thank You very much jjjnettie for the advice. I really appreciate it. Thanks again to Paul and Mario for your thoughts on my problem as well.