Is there any chance you can post a picture of the top mount, if you get chance?
It’s common for the top mounts to have a visible gap, so common I fact I think it’s deemed normal, I don’t think this should be the case. I replaced mine a while back with solid mounts which fasten securely to the turrets, at the time they had a gap which looked like the one in the picture you posted, I don’t recall there being any knocking at the time. If the gap on yours is quite severe then I would be concerned.
Jack the car up on the side you’re experiencing the knocking, check the damper for leaks and check that the springs haven’t snapped, they usually break on the lower 1 or 2 coils. Also check that the springs are seated correctly. Then grab the wheel and rock it side to side, there shouldn’t be any movement, if there’s play then it indicates worn wheel bearings or a loose hub nut, which is unlikely.
The ball joint will cause knocking if it’s worn, this is checked by lightly leavening a pry bay between the hub and ball joint and checking for play.
They’re the basic check I would start with before looking at replacing anything.
