Horn going off continually when car unlocked

2000 Rover 25 with Mk1 160 engine conversion. Lock the car and the horn dies - unlock it and it's constant. I recently replaced the car's battery after the car being off the road for over a year. When I disconnect it the horn obviously stops - reconnect it, and it usually takes a while before it starts off again. Horn works fine individually, and strangely the horn honks in conjunction with the hazard light flashing (this is the only time when the car's on that the horn isn't constant, but it's still on).



I've read that it could be the BCU, but mine doesn't have one, so any alternative suggestions would be appreciated.

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