Horn not working after changing passenger fog light

[AndyC-182]

I've just fitted a new passenger side fog light without any issues, however my horn has stopped working. I've read on another thread it could be something to do with a white cable that the horn is either connected to or earthed by, but I can't see any stray wires near the fog itself.

My car failed its mot yesterday because of this, so any ideas would be appreciated, whilst there is a free retest still on offer by the garage!!

I have a 182 cup if that is important.

Thanks!