Who is responsible for COC

[Dave A]

Dave, are you saying that a CoC is only valid for two years?

No - I'm saying that in terms of the law it may only be deemed valid for a maximum of two years.

The reason I ask is that your statement doesn't make sense...A COC may be deemed valid for as long as there is no changes to the electrical installation... and for a maximum of two years...

Martinco's question was if the COC would still be valid after three years if no alterations had been made. The electrical installation isn't being sold, the insurance merely needs a valid CoC for insurance purposes. It should be easy to answer it with a yes or no.

The legal position is clear. The answer is no.

To try to remove your confusion -

Here's one of the core problems in play - there can be a change to the electrical installation at any time, even within hours!!
For example, the earth leakage unit goes faulty (it happens).
An earth connection is lost (it happens).
It rains, there's moisture ingress into the installation somewhere, and suddenly the insulation impedance reading plummets.

I suggest the way to look at it is there are two "clocks" ticking.
One is time.
The second is events (which could be overt and known, or covert and unnoticed).
Either one trips, and the COC is no longer legally valid.