My Peugeot 207 has a horn fault. When left parked in drive horn will go off and sound constantly.

Many French vehicles made with airbags have issues with the "horn ring" (the part of the steering wheel that provides ground connection for the horn switch, triggering the horn relay). Find the horn relay under the hood of the vehicle, and disconnect it. You should measure 12V at the terminals of the connector (the CONNECTOR not the RELAY itself), but NOT GROUND CONTINUITY (unless you hit the horn button, of course). If you find ground continuity at any of the terminals of the connector, suspect that you have an issue within your steering column.
If you find no ground continuity in the connector, check across the secondary terminals of the RELAY. If you find continuity at the secondaries on the relay, replace the relay.

Check horn relay may need replacing .