Horn sounds continuously

Rover 75 Mk I. Whilst cleaning I left the bonnet and petrol filler flap open, windows wound down, alarm off.



Keys in the car but not in the ignition lock.



No-one near the car, the horn starts to sound continuously and LOUD. Disconnected the good and well-charged battery.



Each time the battery is reconnected the horn sounds.



Only way to quieten the horn is to remove the horn fuse. Removing the alarm/immobiliser fuse had no effect. Horn still sounded.



The car starts every time, no issues, no obvious alarm issues - just the horn sounding.



The only thing I had done was to spray WD40 liberally under the bonnet catches on the scuttle and over the engine and component covers prior to a quick polish with a rag. I doubt this last bit of work is anything to do with it but I though I'd mention it.



The car starts on the button, drives and is fine in all other respects, only the horn seems to be going mental.



Can anyone please help? I have to get the horn to work on a button press for the MOT so I can't run around with the horn fuse removed.



Findon, nr Brighton

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