Capping flue inside loft after removing chimney stack

[xyz123]

Hi, we are in a semi detached house looking to do a loft conversion. As part of that work we are discussing with neighbour to remove "common" chimney stack from both houses. None of the houses use fireplace or plan to in future.

In our house we will remove the stack above roof line and also chimney breast inside loft. In neighbour's house, we will just remove chimney stack above roof line, leaving chimney breasts inside loft.

What do we do with the "flue" that is left inside neighbour's loft? Should it be "capped" somehow or just left open. If capping, can we use standard chimney flue caps to cap it or it needs to anything special.

Thanks for reading my message. I am aware this is subject to party wall agreement and we will get one formally but just trying to sort ouf actual scope of work.

Thanks