So, you've hired an electrician who is also a builder to carry out remedial works. This guy came recommended by your painter and decorator. So far I see nothing wrong with what you've done.
The damp issue is not electrically related so this is the wrong place to discuss it, having said that as the Electrician/Builder (E/B) has represented himself as competent to carry out this work then he is responsible for it being to a reasonable standard which it clearly isn't. Again I can see nothing wrong in what you have done.
This assumes that the work done is by his recommended method using his recommended materials. If on the other hand he is carrying out works to somebody else's specification then that's a different situation.
To clarify, if you have instructed the E/B to board out the wall and make good the electrical issues and not specifically mentioned that there is a damp issue to resolve then you're on sticky ground.
This is quite typical in that work is often tendered without a clear specification in the hope a contractor spends time, effort and money diagnosing problems and proposing solutions.
From a contractors point of view this is frustrating as many unscrupulous clients then put this work product out to tender looking for the lowest possible price.
The electrical issue you've presented us with is trivial and easily remedied and I don't believe is the cause of your conflict, rather an attempt to add supporting evidence to your defense of the E/B's claim.
Other than possibly not being competent to source these kind of works you haven't done a great deal wrong if, and it's the big if, you've told us everything.
I'd personally be reviewing all my communications and making sure all details are correct and that the E/B has definitely failed to deliver against what is written, not what you think should have been done.
Also I'd be communicating with the E/B and attempting to resolve the issues by mutual consent.
Oh and change career, estate agents are scum and it should be held against you in court, in my humble opinion.