Thanks for replying.
I'm sure I'm doing this right (I've been into astrophotography for a few years now, used APT, NINA etc).
But, on the plan screen, this is what I see:

If you look at the times across the bottom, they start and end with midnight. I simply cannot figure out any way to do this differently. I agree that the help pages show plans crossing midnight, but I can't do it!
I'm pretty sure everything else is set up correctly eg date, time, location etc.
Any ideas?