Having fun now!
SGP worked like a charm (finally).
Details:
Each panel 50 X 60s subs gain 140
I was able to halve the processing time by using two computers.
1: Each image set processed separately to produce 4 master images.
2: StarAlign in 'Register/Union Mosaic' working mode.
3: StarAlign in Register/Match Images working mode.
4: Used the registered 4 images in Gradient Merge Mosaic.
But still a lot of work, about 23 hours all up (that includes acquisition, coffee breaks and some sleep :-)
hires:
https://astrob.in/girdmo/0/
Thanks for looking!