This isn't quite what happened ...
You are doing a PIR on a property which has had about three phases of alterations. The householder wants to retain power so that she may use Wifi for her work, so each circuit is locked off individually. Whilst inspecting the downstairs lights, you undo a switch which controls an outdoor luminaire beside a door leading from the dining room to the garden. You get a shock (both literally and figuratively). ?
FI reveals that the lamp was fed from the upstairs lighting circuit.
I think that such a situation is rather dangerous. One might argue that safe isolation should be applied to every accessory, but I think that it would be reasonable for an ordinary person to change a broken switch. It isn't so much a matter of one fault to danger, but one repair to danger.
It also means that if the CU is marked "downstairs lights" and "upstairs lights", the markings are inappropriate.
C2 seems rather extreme - a lot of effort might be required to separate the circuits.
C3 gets my vote.
no code seems reasonable subject to the installation being sound in all other respects.
Interested to hear your views!
