fire break detail is incomplete along the top of the party wall

Bizarrely there is firestopping over the missing blocks.
Doesn't need to be blockwork. Can is a suitable fire stop. The joints in the party wall and poor and I wouldn't consider that adequate either.

Advise you neighbour, as it's a shared wall. They should by rights contribute have to the rectification.

That is it's done, along with some cement and bits of block, in the house I referred to, and badly, as there are still gaps. Not as bad as your pic though. Even with fibreglass in the gaps, it looks wrong compared to where there is block to the eaves.

This house will be sold next year and I figured a diligent surveyor would, if they went through the attic in detail, spot it.

The other solution is to sell it as is. You not required to make good non compliances when selling. The purchaser is aware and can act accordingly.

Do what works best for you.