The problem is we can't use BS7671 definitions as they would class a FCU as a new circuit. ... So in the house all electrical equipment is supplied from the DNO fuse so it is all one big circuit with sub-circuits taken from it. ... OK before you shout I know that's not what it means. But it would be one way of interrupting it. If a FCU is a sub-circuit then why is the consumer unit not also producing sub-circuits? ... So if we can't use BS7671 definitions then it becomes a free for all.
I'm not going to 'shout', because I agree with you. What you're calling a 'free for all' is what I've been calling a 'grey area', where the individual undertaking electrical work has (if(s)he is concerned about 'notifiability') to make a judgement, based on his/her individual interpretation (maybe aided with discussions such as this) of the meaning of 'new circuit'.
I've made my suggestion, which seems to correspond to common sense to me. For a start, I don't think that it's sensible or helpful (in context) to regard the whole of an installation as 'one circuit' because it's all protected by the DNO fuse. Secondly, I only think in terms of 'primary' OPDs - i.e. those in CUs or switch-fuses etc. supplied directly from 'the origin of the installation' (meter in a domestic instal) - hence excluding any 'sub-circuits' potentially created by FCUs, 'sub-CUs' or whatever. That is obviously just
my way of looking at it and, as you will point out, it's not exactly what BS7671 says - but it seems to make pragmatic sense. You, or others, may well have different ideas - and it's for each of us to apply whatever interpretation we feel is appropriate when deciding whether electrical work we do should be regarded as notifiable.
The English language is unlike most others in that the dictionaries show how the word is used rather than what the word means.
To be fair, decent dictionaries usually do both.
Kind Regards, John