How to repair chimney roof leak?

[loserkite]

Hi everyone,

Sorry but total novice here. There is a signifcant gap in my chimney, which I believe is the source of leak of rain into my house

I went up the roof and saw that this gap (see attached photos). Judging from the white residual, it looks like there has been some previous work done, but it has been removed or falled off for some reason.

Looking for advice on how to repair this.

Thanks

[loserkite]

thanks guys