Plumbing is licensed work, and certain types of it generate paperwork that matters later even though it seems like administration on the day.
A written quote before work starts is the first item, setting out labour, parts and anything else so there is a shared understanding of scope.
For notifiable work, a compliance certificate is issued by the licensed plumber and records that the work was carried out to the applicable standards. Gas work in particular is certified, and that certificate is the evidence the installation was done by someone licensed to do it.
Where a fixture or appliance has been installed, the manufacturer's warranty documentation and the installation details matter, because warranty cover often depends on correct installation by a licensed person.
An invoice that describes what was actually done, rather than a single line, is worth having. If a question arises later, whether from an insurer, a buyer or a subsequent tradesperson, a clear record of the fault and the repair is what answers it.
Keeping these together with the property records takes no effort at the time and is difficult to reconstruct afterwards.