On Wednesday, 24 August 2016 15:51:46 UTC+1, David wrote:
Does the fact that an electrician signed off this installation imply that
the rest of the system was safe at the time of the upgrade?

No.

The installation (or minor works) certificate only relates to the new work stated on it.

The electrician should not have *knowingly* reconnected *unsafe* wiring to the new consumer unit, but that does not mean the wiring was safe. In particular there may be defects which were not visible and not apparent on testing, eg bare live wires under the floorboards but which nevertheless pass an insulation test. Old rubber wiring can also pass an insulation test if not disturbed, but can crumble at a gentle touch.

"upgrading the fuses from wire to plug in RCD," doesn't quite make sense either.

Owain