A. Between 10.00 pm and 6.00 am in a built-up area
B. At any time in a built-up area
C. Between 11.30 pm and 7.00 am in a built-up area
D. Between 11.30 pm and 6.00 am on any road
Your horn mustn't be used between 11.30 pm and 7 am in a built-up area or when you're stationary, unless a moving vehicle poses a danger. Its function is to alert other road users to your presence.
A car horn must be secured to the car and be able to be operated by the driver. The usual place for the horn to be activated is on the steering wheel, but it's not necessary for it to be there; it could be on a stalk behind the steering wheel (some older cars have this).
It must produce a continuous note, i.e. a bulb horn is not legal for cars first used after 1 August 1973, and you can't have a horn that sounds like the General Lee from The Dukes of Hazzard.
Pre 1906 vehicles can have a gong, bell or siren audible warning, provided they were designed before 1 January 1905 and constructed before 31 December 1905.
If the horn doesn't meet these requirements then it will fail the MoT.
There are guidelines to MoT testers that the horn should not be 'harsh or grating', but this is subjective. As long as your horn sounds 'horn-like' then it will pass.
A car or motorbike horn is usually between 100-110dB, whereas a truck horn can be up to 120dB. Horns are available that produce more volume than this.