Hypothetical question…
If your horn is found not to be working when you get pulled for a routine but you’ve put it on the defects sheet - would it attract a delayed or instant prohibition, or neither?
cheers
CC
If it was put on the defect sheet before you left base then that would be illegal as you took out a defective vehicle.
If it stopped working once away from base and then defected that would be legal as long as your intention is to get it fixed ASAP
been checked many times but i can’t remember ever being asked to demonstrate the horn was in working order 
VOSA checked mine a few weeks ago. I’ve got an FH12 and the proper horn wasn’t working, just the one that sounds like it belongs on a Mobylette but it was enough to satisfy them. It is a legal requirement so I wouldn’t be surprised if they nicked a driver if the horn was inoperative, especially as they seem to be very fine-orientated at the mo.