In article ,
says...
I will soon be replacing an existing consumer unit with a new unit and the
new unit will be located about 1 metre from its present position.

This will require the various circuit cables to be shortened but no other
changes will be made to circuits.

Also the existing tails from the meter to the new unit will need to be
extended (actually replaced with new) by 1 metre to reach the consumer unit
position. Total length of the new tails will be about 2.5 metres.

Bearing in mind that no circuit changes are to be made, can anybody tell me
if I need to provide some form of certification for the electricity board
people (nPower) who will connect the new tails from the meter to the
consumer unit?

So basically you're going to disconnect eveything, modify the wiring and
then reconnect it all again - nothing much to go wrong there then.