Horn stops

[Knucklerash]

Hi all wonder if anyone had any ideas,
My LML star is in for its MOT, I have Done all I need for it to pass but the Horn has stopped or stopping working. I start the scooter up and the horn works fine put it in gear and ride off and it stops until I stop and restart then same again. It's a brand new battery that's fitted but it still stops if I disconnected the battery and run it without, I'm flummoxed