Yes.
What I meant was that if your neighbour has already extended the party wall, if you were planning to build off this, you would have to pay your neighbour half of the build cost of the extended party wall for the right. If there is a generic and published way to calculate this, I'm gutted I missed it - if not, isn't an independent surveyor the only way? Happy to be wrong on this one - not very satisfying at all to pay of that expense.