how do you install a new (not replacement) CU without installing any new circuits? Is it installed?
Well, if I thought as you seem to think, by 'transferring' some existing circuits from an existing CU (which was going to continue to exist).
Consider a perfectly credible (albeit not very sensible) hypothetical scenario ... one has an old CU without any RCDs and wants to provide RCD protection for some of the circuits. One decides to install a new small CU, with RCD's, and transfers some of the existing circuits to that new CU, but leaving some (maybe cooker, immersion and perhaps one lighting circuit) still running from the 'existing' CU. One has not replaced a CU and, by your reasoning, has not created any new circuits - so, in terms of what the legislation actually says, the work would not be notifiable, would it?
However, this is a rather silly side discussion. I was merely trying to illustrate that this part of the legislation is badly written. They obviously should have (as in your other document) included the installation of new, as well as replacement, CUs. They also should have been far more explicit about what they mean by "the installation of a new circuit", so as to avoid uncertainties and discussions such as we are having. Don't forget that, as often discussed here, in terms of the BS7671 definition of 'a circuit', adding a FCU (and something it feeds) to an existing circuit constitutes the creation of a new circuit, and I don't think anyone really believes that they intended such work to be notifiable - after all, adding an FCU to an existing circuit was one of the few things that was
not notifiable prior to April 2013 (and still today in Wales).
Kind Regards, John