Scott
Thanks.
I tried ISO1600 initially at 3 min. That didn't lift the histogram much off the left. But still, the centre of eta carina looked overexposure on the camera's LCD. That worried me, so I dropped it to 800. So I guess that you can still go deep without skyglow at Lostock.
At home, without the filter, 3 min reaches the skyglow easily, but I have not tried it with the filter.