As with other replies, before any criticism I told the OP how to find the fault.
You did, indeed, as did others - and it surprised me a little to see those responses without any 'reservations' or criticisms about the work that had been done (and the implicit absence of pre-energisation testing, let alone notification). I personally resisted the temptation to respond to the OP's question, since I was in two minds about the pros and cons of advising someone in relation to work I did not feel they should be doing.
It's probably apparent that my mind is 'all over the place' with this one, since one can view it at various different levels. In some senses, the OP is the victim of his/her own honesty. Had we been given no background, and had the question just been "a lighting circuit has developed a N-E fault; what is the best way of locating the fault?", then most of the tangential discussion would probably never have arisen. However, given the background which was provided (and given 'research' on the OP) it is indeed probably true that (s)he should not have been changing a CU (which was very probably done without notification), and certainly should not have changed a CU without at least pre-energisation testing
Rather than replying with an acknowledgment and, for example, 'thanks, I'll do that' he came back with "Other test results to this point are irrelevant" (strange way to spell non-existent).
As I've said, I was moved to comment because I don't really like the 'language technique' which was used - which I suppose one could describe as condescending or patronising. As for the the OP's response, I would say that it was perfectly reasonable for someone who was focussed on the specific problem/question in hand. If a N-E fault had been detected, the results of any other 'standard tests', undertaken before or after the CU change, would not help in locating/elucidating the fault - so I would think that (in terms of person focussed on the specific fault about which questions had been asked) "irrelevant" is not an unreasonable way to spell "non-existant".
This is not a DIYer trying to learn or save money by doing it himself. I don't think I will be proved wrong by saying he is a plumber who, presumably, said to his boss that he could do this job. Now he is stuck. He can change a consumer unit but does not know how to find a fault.
Yes, I think that's all probably true - but I have to say that I don't research the background of posters before responding to them (not that I have responded to the OP in this thread) - I simply take their posts/questions at face value. However, it is undoubtedly true that, quite apart from laws, regulations, knowledge and competence (to do the actual CU change), no-one (not even a 'qualified electrician') should be changing a CU if they do not feel confident that they can deal with any problems that the work might uncover or cause - hence my failure (as explained above) to respond to the OP's question.
Has BAS not noticed this thread?
Kind Regards, John.