In case anyone else gets party wall notification. I had one for a wall behind my garage, it was simply to build a soakaway and I was entitled to employ my own surveyor, at no cost to me. I did that and the paperwork was signed off without any stress to me. It is a surveyor`s expertise, that was needed, not mine.
In Allsorts case, I would also have employed a solicitor. When the op sells, the buyers solicitor will want to see all the paperwork and agreements. To sell a property these days, a very complete file is needed. Employing a solicitor would have ensured that all the paperwork is in place