On Mon, 29 Jan 2018 23:54:20 +0000, Roger Hayter wrote:

My electrician, who may be a bit of a literalist, said that if he were
to make any modification to my installation (other than what he did,
which was to wire an additional consumer unit directly from the terminal
blocks connected to the meter) then he would have to test every part of
it, to ensure that it was safe. Apart from the inordinate amount of
time this would have taken, a few minor alterations would have been
necessary. This would have taken somewhat more than five minutes.
Especially putting sleeving on about half the earth wires in the house.

He is wrong.
New work has to comply with current standards. The bonding and earthing
must be checked too to ensure they comply. If the houseowner doesnt want
an undersized (say) earth cable replacing while you fit a shower, then
there is nothing you can do, you just note on the installation
certificate that 'customer did not allow upgrading of earthing, which is
undersized original'.
That is the electricians bit done.

As for new push in CBs in a rewirable fusebox, I would say no Part P
notification, as it isnt a new circuit, it is an amendment of an existing
circuit.