Treat it as a formal process from the moment it arrives, even if your instinct is to explain why the complaint is wrong. Acknowledge it in writing, follow a clear procedure, investigate where needed, hold a meeting to hear them out, give a reasoned outcome, and allow a right of appeal.
Done properly, a grievance process protects you even if the complaint does not really stack up. Brushed aside, a weak complaint can still become a costly claim, because tribunals look at how you responded, not just at what was raised.
Every situation has its own details, and those details change the right answer. For a read on yours, take the free Employee Situation Check or book a call.
General guidance to help you decide your next step, not legal advice. Employment law changes, so always check the current position for your own situation.