Think I've discovered the problem.
I tested the battery on a mates battery tester and it showed 100% health and 100% charged. Output was 12.7 volts 263 CCA.
So then got myself a digital multimeter and checked the output at the horn connectors. It showed at 11.8 volts. Denali state the horn requires 11.7 volts and bearing in mind the horn is on the same circuit as the side lights, tail light and my led spotlights, that explain why horn just clicks after 6 or 7 presses of the horn button when the engine isn't running.
Checking mot regs it says if the battery isn't fully charged the engine can be run to check and pass the horn.
If theres a reasonable cost solution to removing the issue I'll get it done, but otherwise will leave as is.
After all dont know why I wanted a louder horn anyway, it's not like how many times do you use them 🤣 I think the lock down is getting to me now, I might have to do an Aldi grocery run on it😄