Stood in the loft and taken some pics out the Velux and actually the pointing isn't as bad as I thought. So it's really a matter of removing and replacing the top cement.
What's good for me, is that there are panels with mounts and rails, on both sides of the roof making access and 'safety'(?) a little better. Not bad locations to attach straps. I have sprayed the chimney with weed killer in the past, but obviously losing that battle.

Quick snap of the gear I bought for ~£35,

[Really need to get a new carpet for the loft, that was a cheap end of roll I bought 25yrs ago after converting the loft.]