Three Oaks Limited review: Incomplete job - toilet blockage

Thank you for sending an engineer to unblock our toilet recently. Unfortunately, the blockage has returned on and off since the visit, which suggests the underlying problem further down the pipe hasn't been fully cleared.
The engineer advised that the issue is due to incorrect installation. However, the toilet has been in use without any blockage for the past twenty-five years, so we're struggling to understand how installation could be the cause now. A fault present since installation would usually have shown up long before this. (Could the engineer be mistaken?)
We'd be grateful if the engineer could return to investigate the blockage further down the line, as we don't believe the root cause has yet been identified. Could you let us know when this can be arranged?