Headlights come on when turned off

With the ignition on, the engine running and the lights turned off, the headlights turn themselves on after about 20 seconds. When this happens, the indicators and horn stop working.

If the sidelights are then turned on, the indicators and horn start working reliably.

If the headlights are turned on, the indicators and horn also work reliably.

If the lights are now turned off again, they initially turn off, but come on by themselves again after about 20 seconds, and the indicators and horn stop working again.

This fault is consistent and can be repeated numerous times.

When I take the keys out of the ignition the lights go out, but when the door is opened, the warning buzzer sounds as though the lights are still on.

Any help would be greatly appreciated.