Daylight in roof slates, what to do???

Newhouse54

I have recently had a few slates replacing on my victorian property. There is no felt, just wooden timbers and slate. My roofer said I didn't need a couple replacing even though there was a thin slither of daylight coming through. The gap ran about 4 inches between slates but only a few mm or maybe a mm wide, nevertheless you can see daylight. He said the water won't get in even though I can see daylight so they dont need replacing. Somethinh about them overlapping and rain wont get in. Now my logic tells me if theres daylight (hole) water will surely get in and clearly if i can see daylight the slates are not overlapped. Is this right?

Gloomendoom

I have thousands of plain tiles on my house. I can see more than a sliver of daylight through every single one of them.

The roof doesn't leak.