Stop repeated use
Avoid flushing or adding more water to demonstrate the problem. Describe whether the toilet, basin or another connected fixture first showed the change.
Plan an assessment
The model, discharge route and access help establish suitable work. An engineer needs to identify the cause rather than assuming a running unit is healthy or that a complete replacement is necessary.
A useful distinction
If one connected fixture still clears while another backs up, include that distinction. It may help locate the relevant part of the route, although shared pipework still needs inspection before the cause is established.