SOLVED
New coupler arrived today.
Thankyou Andy (Sir Trikealot)
. Thankyou Simon for your helpful posts.
Fitted 3 hrs ago and not a peep nor a beep from the horn unless buttons are pressed. So normal service resumes and car is 100% problem free, for now at least.
As for the cause, short of a Crime lab I can't be 100% sure. I'm pretty convinced it is brake fluid. As for how it got there, I have no idea. Ive owned the car 10 yrs, I do my own maintenance and repairs, it is left with same trusted garage for MOT only, every year.
Lee.