What's a mot fail?
Mine has passed both its mot's with the awwooogggaaa horn as the only one installed
I have copied the mot info from the .gov website - it would appear 01 Aug 1973 or earlier and you would be fine
MOT Test of Horn
Operation
Control can be easily reached by the driver
Loud enough
Not a sequential multi-tone.
Further Information
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.
Reason for Rejection
The horn control missing, insecure or not readily accessible to the driver.
Note: Inform the driver if the horn is insecure.
Or the horn is
inoperative
is not loud enough to be heard by another road user
is a gong, bell or siren
has more than one tone
Note: When operated, a two or more tone horn automatically produces a sound which alternates at regular intervals between fixed notes
If fitted to a vehicle first used on or after 1 August 1973, emits a sound which is
Not continuous or uniform
Harsh or grating