Just been changing the oil (like an idiot I ordered the wrong filter size, so I've got clean oil but old filter . . . also was trying to spin it the wrong way for 20 mins before it clicked I was doing it wrong )

Was about to do the transmission fluid and noticed a hole when the fly wheel(?) is

Looks and feels perfectly round so I assume it should have some kind of bung / cover in there? I've put 5000 miles on it since buying and can't say I've noticed anything wrong.

Should it have a cover?

That looks like an ideal candidate for duct tape "repair"

Looks like an inspection hole to see the TDC mark on the flywheel. There's probably supposed to be a rubber bung in it to prevent dirt ingress.
.