Non-complaint installation despite certificate issued

High speed gas sold me and installed a gas heater in Killarney, Johannesburg, 2019.

They installed a new valve for the heater, meaning that there is one valve for each appliance, heater and existing stove. They issued a certificate of compliance for the work.

Now, two years later, I am selling my flat and asked them for a certificate of compliance. They say it is not compliant, as a valve for each appliance and then one that can shut off both appliances is needed.

They have now quoted me to fix their non-compliant installation, saying the "portion of the installation that we certified was done correctly".

I phoned to try an reasonably explain the situation, asking them to fix their non-complaint installation for free, but they have refused. I was of course willing to pay for the new certificate of compliance.

I will not pay them to fix their non compliant installation, I will go to another service provider.