Hi I recently got to visit Grain Fort in the isle of Grain in Kent
My photos for some reason have a faint black circle in them and I don’t know why?
I see it on the first - against the blue sky/sea on the horizon above the fort.
No idea what it will be - beyond something on the lens?
Check if it’s still there now you’re at home …. against something like a sheet of plain white paper.
Had the same with my brand new DSLR - it was a similar large circular mark that I thought might be dirt in between lens elements - but relieved to find it was the same with other lenses. Never did discover what it was … if was gone the next day and never returned. 
Hmm maybe the lens is dirty?