You can quote all the reg's and theories you like, but at the end of the day, when it comes down to actually operating in the real world, you have to be practical. Around my way there is an estate of around 2000 houses, and about 5 local sparks all competing for the work. These is almost an "accepted price" for a CU change, all the houses are virtually the same, all had 3036 boards complete with a borrowed neutral for the landing light. You don't need to test it first, it just has. I can guarantee that if you turned up and started recommending an initial EICR or factoring in sorting out the neutral problem you would be treated with suspicion and not get the work. Most customers with an ounce of common sense will get 3 quotes and its almost always either the cheapest or the one that can start first that gets the job. If you have the luxury of sticking rigidly to the letter of the law then good for you I say, but you wouldn't get many CU changes round my neck of the woods. That's life, as Ester used to say.